Strange 'clunk' sound while idling?

This one has me stumped.
While idling in park I can hear a strange 'clunk' sound about every 8 seconds or so.
It doesn't seem to do this when idling in drive w/ foot on the brake.
I'm absolutely unclear as to what the hell this is.
Perhaps it's a solenoid of some sort actuating, it does not change frequency, volume, or tone w/ RPM.

There also sounds like a tick in the valvetrain, but that's another topic. I'm talking about the 'clunk' at 3 seconds and again at 11 seconds.



Any guesses?

MountaineerTom said:

Do you have A/C? Could it be your compressor kicking on?

Click to expand...

I thought that might be it as well. The fan was off, but the AC switch was in the recirc position.
I'm guessing that was it.