this is a long post but i am trying to be detailed to help expedite responses here. i have a 2000 gmc jimmy 4 dr 2 wheel drive with 235k miles on it. I just rebuilt the motor and while I had everything out I replaced the entire refrigerant portion of the a/c since my old compressor was rattling and on its last legs it sounded like. the old orifice tube had the screen broken and little chips of metal in it. the bottom half of the old evaporator was plugged with leaves/dirt/bugs also... beyond cleaning.

So its a new Delphi compressor, new suction/discharge lines from the compressor, new evaporator, new condenser, new dryer, and new hard line from the condenser to the evaporator. Also new low pressure switch. All parts are a/c delco except the compressor which is Delphi. The evaporator came with an orifice tube pre-installed and oil in it already. I added PAG 150 oil in the prescribed amounts (from alldata) to each of the components (except the evap which came with oil already). It was 9 oz total oil and working off memory here it was 2 in the compressor, 3 in the evap, 2 in the condenser and 1 oz each in the soft lines and dryer.

Anyways, its all back together and on the road and I took it to a local shop to have the system evacuated/charged with r134 since I don't have the equipment to do that. they evacuated and then added 1 lb of r134 to the system (all data says it should hold 1.8 lbs), i'm not sure if they were rounding to 1 lb or if it actually took 1 lb. the a/c blows super cold though and works really well, however I noticed a low pitch whine/hum that only occurred when the a/c was on and generally under 1000-1200 rpm. I notice it idling at a stop light and when i'm slowing down approaching a stop when the rpms drop down low. I can feel a little vibration in the steering wheel also that coincides with the noise. the noise only occurs when the compressor is engaged. a/c off = no noise.

so i ran the a/c while it was parked and felt around to try to see where the noise was coming from, and when you put your hand on the discharge hose from the compressor its vibrating like crazy when the compressor is engaged, you can't see it, but you can feel it. you can also kind of hear the hum/whine if you listen close over the engine noise. on the dryer side of the evaporator you don't feel anything, just that the dryer is super cold like it should be. i also notice that when the a/c clutch is engaged, the belt tensioner jumps around a lot. as soon as the clutch disengages the tensioner is almost as smooth as a top. no vibration of the discharge hose either. so there is definitely some uneven loading on the belt by the compressor. all the pulleys look smooth too, nothing bent or oscillating with rotation it doesn't look like.

i hooked up my low side pressure gauge (i just have a r134 bottle attachment with low side guage on it) and the low side pressure is in the mid 30's at idle rpm and ~90 deg air temp, which is around where the generic chart on the gauge said it should be. rev up to 1500 and it drops to the low 30's. nothing too out of the ordinary from what i can tell.

my question is what could be causing the compressor to vibrate/whine like that? its all brand new and brand name. the compressor doesn't rattle like its about to throw a rod like the old worn out one did, it sounds smooth mechanically like it should, but its like a vibration/hum caused by fluid flow at high frequency that it sounds like. I've got about 400 miles on the new motor, and 250 miles since the a/c was charged up. i cant hear the hum or feel the vibration when i'm driving because it seems to go away above ~1200 rpm. something is not right though. too much oil in the system? it cools really well for being 90-95 deg outside though. any thoughts?

 

Two thinngs come to mind. When the compressor engages it puts a load on the harmonic balancer and all associted brakets. Assuming the balancer is sound and brackets are tight the New ac compressor could be faulty.

 

The balancer is new, part of the engine rebuild just completed with the new a/c.

One other observation was that I was able to turn the a/c clutch by hand when I was adding the oil to it to circulate it.