Help me diagnose a front end noise

89 Saab 900 recently started giving a sort of popping sound when launching. Just a single pop as the clutch is engaged and torque is applied. No other noises that I can hear, and the car drives normally. It's somewhat inconsistent, but it feels like the more the front end lifts (as when launching on an upward slope) the more pronounced the pop. In these situations I can feel the pop as well.

I pulled the front wheels this morning and didn't find anything obviously wrong. I know these cars are known for rust eating the front suspension mounting points, but the right side was comprehensively repaired some years back (not by me), and the left side seems solid. I replaced the lower control arms last year as a precaution, and did the bushings, along with the sway bar bushings, as well. For those unfamiliar, it's a simple double-A arm setup.

The only things I've done recently are to swap out the snow tires for the summers, and pressure wash the underside to get rid of the winter muck. Thought it might be related to the wheels, but the lugs were tight and everything seems as it should be.

What should I be looking at next? Ball joints? Brakes?

My first thought is CV joint but it could be suspension rather than driveline.  To determine which it is try bouncing and/or lifting the front end.  It might take two people to get enough motion.

My truck has started doing this. I'm pretty sure it's an engine mount. You might put those on your list to check as well. 

Front engine mount?

If you have an S model, then if memory serves, it's got hydraulic mounts. The front one fails, and the engine lifts until it smacks into a u-shaped travel limiter.

Pop the hood, start the car, set the e-brake, and watch to see if the engine rises when you let the clutch out.

If this is, in fact, the trouble, then when you replace the mount, take a few minutes to keep it from failing again. Find an old length of rubber hose, the wall thickness of which is about the same as the space between the top of the mount and the bracket above it. Cut a length of hose that's as long as the inner width of the limiter, then slice the hose lengthwise. You now have a small, rubber "hotdog bun."

Slide the bun under the bracket so the bun takes up the space between it and the mount, with the slice on top. Hold it in place with a couple hose clamps you have kicking around. The bracket also has a couple small holes in it, so if you wanted to drill through the rubber and use the holes to keep it in place, that'd work, too.

All that said, if all you can get nowadays are fake hydraulic mounts that are actually solid rubber, I wouldn't bother with the hose fix.

Oh, the limiter has a couple spacers that are sometimes keen to roam when you remove the bracket. Kind of a pain if you lose one.

I will look at the engine mounts. I did consider this, but I replaced the front one last year. The front is the non-hydraulic type, but the right and left ones appear to be hydraulic. They were not showing any leakage last I checked, but I will look again.

CV could be it, as it's certainly being loaded when launching, but my understanding of CVs suggests that it would also produce noise when turning - am I incorrect in that? FWIW, I did regrease the CVs and replace the boots when I replaced the tripod bearings and one wheel bearing soon after I bought the car.

02Pilot said:

I will look at the engine mounts. I did consider this, but I replaced the front one last year. The front is the non-hydraulic type, but the right and left ones appear to be hydraulic. They were not showing any leakage last I checked, but I will look again.

CV could be it, as it's certainly being loaded when launching, but my understanding of CVs suggests that it would also produce noise when turning - am I incorrect in that? FWIW, I did regrease the CVs and replace the boots when I replaced the tripod bearings and one wheel bearing soon after I bought the car.

Inner CV joints rarely go bad but when they do they don't often make noise when turning.  Outers should make noise when turning but not all of them have read the rules.

I came here to mention the front motor mount.

Bad outer swaybar bushing? The way aftermarket parts for these cars are getting so E36 M3ty now, maybe one of your new ones has crapped out already??

Bad shock bushing? If bouncing the car doesn't make the sound, and the lowers look good, then inspect the uppers by jacking up the suspension to compress the spring -- the shocks limit travel, so the upper bushings will be compressed when wheels are in the air.

If it is a bad shock bushing, and if you didn't see a torque spec for the lower mounts when you replaced the lower A-arms, find that spec and heed it: if the lower mount nut is overtightened, the threaded mount piece can break off.

A bit more fun: the paired upper shock nuts are sometimes fairly narrow. I ground down a cheap wrench in order to fit the lower nut while tightening the upper one against it. Or you may have just single nylocks.

Skip polyurethane lower bushings -- they get pummeled and ovalize pretty quickly.

And after all that...bad ball joint?

I also think that it could be a worn CV joint. But I'd also check the other things mentioned by various GRM members. Btw, how many miles does your Saab have on it?

Took a quick look at things this morning. Checked the motor mounts, shock bushings, and sway bar bushings - all OK. Outer CVs have similar play on both sides (checked both wheels straight and at full lock), and only a small amount; inner joints (tripod bearings) are new last year. Ball joints are quite difficult to check, but from what little I could discern they didn't seem to show much play. The only thing I did find was that the right side shock top nut needed a full turn or so to snug down (they are single nylocs; shocks and bushings were new last year) - that could be the issue, but I haven't had time to test drive yet. If it isn't, I probably have to replace the ball joints, as they are one of the few things in the front end I haven't touched. I hate throwing parts at it without knowing, though.

Mileage is unknown, as the odometer is intermittent, but it showed ~125k when I bought it. Judging by the interior condition it's not likely to have huge mileage on it.

Finally test drove it. Seemed better initially, but I suspect that was just wishful thinking. I did several abrupt on-off throttle transitions to try to provoke the CVs, and didn't hear anything. Nonetheless, it's hard to repeat consistently on the road, so I started experimenting in the driveway. If I reverse and brake abruptly, the sound will appear consistently when I start forward. All of this seems to suggest an up-and-down motion in whatever the source is, which points to ball joints, I think (given the other stuff seems to check out OK).

There's a pretty wide range of ball joint options available. Aside from avoiding the bottom of the barrel, any recommendations?

Reverse to a hard brake may also slide the suspension arms on their bushings. You might also check the subframe bolts, if it has one, and make sure they are all tight. 

There's no subframe - the A-arm bushings are in carriers that bolt to the unibody. I will take a look at the bolts holding the A-arms in place.