Since today was one of the only nice days (ever) in Seattle, I had the windows down and several times heard a noise, which sounded like a knock, which I've never heard before.
When driving next to a building or row of cars (when you can hear the sound of your engine bouncing back towards you) I noticed this sound faintly. It sounded like a diesel school bus or a rattlesnake. I was always down around 1-2.5k rpm, and it was more evident when going up hill. Once I got into the higher RPM range or in boost, I did not hear it at all. Other than my car seeming pretty sluggish these days, nothing else appears to be wrong, besides a vibration at idle. I believe this may be related to my cam tensioner (seems a little noisy) or breather system.
The ambient temp was about 80 degrees. As noted above, the noise was in low RPM and (probably) when I was in a lower gear than I should have been. My car is at 146k miles. I've never had a CEL over the past 15k miles I've owned it. I run German Castrol 0w30 and am at 5500 mi on this change (blackstone guys suggested 6k miles this time). I have about 200 miles on new NGK iridium plugs gapped to .032". My valve covers are leaky in the plug wells.
Those are all the details I have that I feel like may be related. Was this just from a slight drop in oil pressure, due to low RPM and high load? Am I being overly cautious?