AC Compressor

Had ac compressor replaced on 2002 Toyota Camry at a shop that works only on imports and has been in business 30 yrs… New compressor runs and cools well. When inside car and compressor is engaged, there is a humming/whinning noise that speeds up/down when accelerator is pressed/released. If I click compressor off or it disengages, you cannot hear noise. The noise can be heard from outside when compressor is engaged with hood up but it’s not as loud (don’t know if running engine is muffling some of noise)and can’t tell where it’s coming from. I took it back to shop. He could not locate where it was coming from. Said he took serpentine belt loose and checked pulleys. He did not think it was the new compressor but did decide to get another compressor. Second one runs and cools good but there is still the same noise. He let me listen to the new compressor and existing alternator with a mechanic’s stethoscope. Compressor sounded smooth when engaged but the alternator was much louder through the stethoscope when compressor engaged. He now thinks the noise is coming from the alternator. He thinks the new compressor when engaged is putting more of a draw on the alternator which has a 170,000 miles on it. I asked if he was telling me that I needed a new alternator. He said no as far as functioning of it. But if the noise is coming from it, that’s the way to get rid of noise. I was referred to this shop by three different I work with. Has anyone ever ran into such situation and what is opionion.

The alternator can be “electrically” isolated to see if it’s making the noise. I don’t know why a good mechanic never thought of it. Just unplug the alternator, start the engine and kick on the AC. If the noise is not there just replace the alternator. Don’t run the vehicle very long with the alternator disconnected. Unplugging the alternator takes any chance of a draw away from it. It’s just an idler pulley when disconnected.

Has he not considered the possibility of a noisy belt tensioner, etc. that is reverberating through the engine block? At 8 years and 170k miles I don’t see how the tensioner could not have some wear on it and sometimes feel may not reveal much.

I don’t buy the alternator draw theory at all although worn alternator bearings could be behind this problem.
Hopefully this is not an A/C pressure problem caused by some overlooked fault in the A/C system. (Much like you might get with a vibrating water pipe in a home, etc.)