Thanks to those who responded.

I will see if I can pick out the sound. I did notice that the sound is still there (just much less noticeable) when the engine is warm. The sound does not seem to vary with engine RPM.

I think I may have figured out what caused the problem, though. I had the A/C recharged last summer (there was no refrigerant in it whatsoever), but during the last month and a half the compressor has been clicking on and off at an increasing rate--even when the A/C is set to Off! I imagine that this has been a pretty significant strain on the serp. belt and/or pulleys. I'll create another thread about this issue--hopefully someone can shine some light on why my A/C seems to be running constantly. This A/C constantly on problem does seem to occur only on highway driving, which is even more perplexing to me.