09 Nissan Cube Drive Belt Tensioner Rattles While Sitting

A week or so ago I noticed that when I put my Cube in reverse, or drive, and don’t move, the drive belt tensioner seems to rattle/oscillate loudly. The sound will disappear as soon as a load is put on the engine. After driving for a few minutes and coming to stop (while in gear still) the sound is gone.

I ordered a replacement serpentine belt and tensioner and replaced the belt yesterday. I manually tested the old tensioner and compared it’s smoothness in spinning and movement with the new one, and there is no difference – the old one still spins smoothly and has just the same amount of play as the new one. I haven’t replaced the tensioner yet because I’m not certain that is what’s causing my problem, and also I don’t have a way to support the engine since I can’t actually get it off without dropping the engine (thanks Nissan).

Does anyone have any ideas what it could be? I plan on having a local mechanic take a look, but before I do I wanted to have a better idea of what it could be. I was thinking it might be a spark plug issue, if there aren’t enough RPMs at idle, but the rattling stops after driving. I did notice once on my OBD2 reader (using Torque Pro) that there was an issue with the evap system, but I only saw that come up once on the test results, it’s passing all tests currently.

Car specs are:

2009 Nissan Cube 1.8L
230,000mi
Idle RPM: 950(approx, at idle it sits about 1 needle-width below 1000)
MPG: 29mpg average (most of my driving is highway, 76mi/day)
Temp: within spec, no overheating
Other: no misfires on the ECU, no CEL, no warning lights on dash, all tests passed on OBD2 reader

TLDR: tensioner rattles and oscillates when not moving at first, then stops after being driven for a minute or so.