Ford Focus AC issue

So, some of you may have seen my off-topic thread about the heat pump at the house going on the fritz - well, now the car's AC has decided to go on strike too. Awesome. The car is a 2005 Focus ST, with about 90k miles on it. I went to use the air conditioner yesterday afternoon and noticed that it is blowing warm air. In addition, there's a new strange noise. Here's what I've been able to figure out about it:

• The noise is rpm-dependent - i.e., it changes volume and pitch in the same way the engine does.
• The noise is intermittent.
• It sounds kind of like the Covenant Ghost from Halo.
• The noise is overwhelmed by other noises (engine and wind) above about 40 mph.
• The AC is not completely dead - some cooling occurs, but only at speed. Below 30 mph, it's like the AC isn't even on.
• If I turn the AC off, the noise goes away completely and everything is back to normal.
• It started on startup after a 100-mile or so trip. Everything was great before then.

Any ideas as to the cause of the noise?

In reply to szeis4cookie:

Either the electric clutch is not engaging properly or the compressor itself is toast.

AC system is low on refrigerant and oil. They'll growl when they are low on oil. An oil/R134 charge might take care of it, but you've got a leak somewhere. Look for oil around the AC lines.

Toyman, we did find a leak in the line that connects the condenser and the compressor. Any ideas on getting at it? One end is easy to get at, but the other is way in the middle somewhere...