I have a 2003 Hummer H2 with 168,000 miles - everything stock except for 6" lift and 38" tires (and it's on its 3rd GM transmission, but that was all before I owned it). After a snow storm a couple months ago, I was washing the salt from the underneath. That night, the temp dropped below freezing. The next day, I started hearing a high-pitched chirping or squeaking sound when driving. It does not make noise at idle. It only seems to make noise when I have my foot on the accelerator. When I let off the accelerator, the sound seems to go away, or at least become inaudible (I have noisy tires). When driving with the squeaking, I can drop the trans into neutral and rev the engine without hearing the noise. The noise does not seem to follow engine speed, nor wheel speed. The only thing I can think of is the torque converter, but I am not familiar with auto transmissions. Anyone have any ideas?

 

You can check the torque converter to flywheel bolts to makes sure they are tight. If they are I would look close at the idler and tensioner pulleys under the hood.