Just wondering if an air compressor should already be going out (bad bearing) in a 2003 Focus ZX3 with 50,000 miles? Is it worth replacing?

If the bearing in the clutch pulley is bad, just replace it. If the bearing is okay, there is no need to do anything.

Put 12 volts directly to the compressor to make sure it is bad
& be sure it has a clean ground

What is the part number of the serpentine belt that bypasses ac compressor on a 2005 Ford Focus

big end bearings( lower engine rod bearings to confuse) knock at idle and get louder with rpm
if the noise changes with rpm and you are hearing it near the ac compressor then it could be the ac pulley bearing
test if it is the compressor by listening and then turning the ac off the pulley will still spin but the compressor stops
if the noise remains , replace the pulley bearings

there is a magnetic clutch located on the front of a/c compressor it spins on a bearing that could be bad if the clutch itself is causing noise from poor engagement then there generally is a underlying problem if air gap on clutch to large causing poor engagement then bearing is usually bad to much shaft end play and so on this could be the signs of compressor going bad low oil in compressors tend to cause noises but when compressors get noisey they have to be replaced not repaired can get expensive fast

This is not a recurring problem for this truck, you just got a bad starter, get a good starter like from NAPA and you wont have to deal with this problem again.

if its paid off then its worth sticking some money into it. I think that you may have a bad compressor pump. Thats what your problem sounds like to me

I take it you have tried a new belt. The Air-conditioner belt driven clutch and bearing repair will cost you nearly as much as a re-conditoned unit. The time, effort and agony is just not worth the trouble of going through any clutch / bearing replacement attempt. (been there done that! And had the right tools!) Ended up replacing the compressor anyway as the internal rollers of the Compressor were the actual problem.

If you have rear drums on your focus then chances are you just need to remove the dust shield (metal cap covering nut) and remove the 30mm nut on the end of the knuckle. at that point you may as well just replace the drums (they come with new bearings).
New drums are cheap 20-50 bux. If for whatever reason you want to just replace bearings make sure you remove the snap ring on the inner side of the drum...

can someone tell me if a squeaky noise when I turn on my ac in my yukon, means the air compressor is going out? Please help!