Alternator or AC compressor noise?
 

I drive a '99 F150 4x4 5.4 EFI 196,000 miles

Recently I started noticing a growling noise that followed engine speed. I immediately thought alternator going out. So I pulled the belt off and it spins free and doesn't feel rough at all.

So I started it up and realized the noise got a lot louder when the ac clutch cycled. Now I'm thinking ac compressor. A short trip to my mechanic who didn't even really look at it very long and he said the compressor is about ready to seize up. Oh yeah and he said $700 too.

I don't have the $700 so I thought I might just put on an ac bypass pulley to get by for a while. Not really a sound decision in the middle of July, I'm thinking, but I'm poor and it beats having the compressor lock up, right?

Today, when I get off work and pop the hood with the ac off and the clutch not engaged the noise is worse than ever. I put a stethoscope (long screwdriver, I told you I was poor) on the alternator and it is making all of the noise. Turned the ac on again and it was really making noise now. So I applied the stetho-driver to the ac compressor and it is quiet as can be.

I am cautiously optimistic that the alternator is the sole culprit and that it only gets louder when the ac cycles because of the extra drag on the belt.

Does that seem like a logical explanation?

Should I just try the alternator?

Do you think I need a new mechanic?

Sorry for the long winded story but I really don't want to destroy a perfectly functioning ac system only to find out that it was OK to begin with and I don't want to buy a $150 alternator for no reason either.

Looking forward to your response(s)

My sons 2006 Navigator had a similar noise. He brought it over to check the alternator and AC pump... With a 3 ft piece of 1/2 inch conduit I used as the steth scope, we found one of the three idler pulleys had a bad bearing.......
Summary is it can be ANYTHING that is spinning. SOMEBODY needs to use a steth ( or similar) and narrow down the location and replace it. Start with the MOST NOISE. It is normally VERY OBVIOUS what the problem is when you use a tube / scope.
About your mechanic, yea he is lacking in knowledge. The compressor ONLY makes noise when it is running, the electric clutch ONLY makes noise when the compressor is off. Alternators and pulleys will make noise all the time, and change pitch when loaded.

I have seen the idler pulleys make plenty of noise before....like steve said, start with the one that's making the most noise and go from there

Had same type of noise. Changed out the alternator because the noise sounded like it was coming from that area. The new rebuilt alternator is louder than the old one. Still had a squeal on startup ,changed out the tensioner pulley, no change. Replaced the belt, the squeal has stopped but now I have a loud alternator. Some days You can't win. Scott
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