So both of my HHRs have done this. I turn the air conditioning on, and at a stop or a light, when I initially press the accelerator, the engine makes a chirp sound. Almost like a knock or misfire. It's only at low rpms and is only there for a fraction of a second.

I feel like its normal with the extra load on the engine, just looking to see if anyone else has this

 

How many miles on each, because you might be hearing the serpentine belt slipping under the load from the compressor. Or the compressor clutch might chatter a bit as the revs come up from idle.

 

62k on the SS. Had about 50k on the old 2.4. Doesn't sound like a belt slipping. Chatter is more like it

 

My first thought was a belt noise also.