Engine idle is under ECM control, there is no "idle sensor" per se.
Assuming you have manual climate control, the A/C operates in defog & defrost modes only. So when you switch to either floor, bi-level or dash vents mode & the A/C button isn't engaged, the compressor won't be engaged. I probably don't need to tell you that localizing a particular sound in the engine compartment can be a real challenge, even for a pro.
If the noise is loud & distinct enough, you could try uploading a video of it to youtube so we could hear it for ourselves.