Lately I am hearing what sounds like a resonance buzz for a short time during the fan power up and to a lesser extent during the power down of the fan. The fan starts up with no buzz but when it reaches a certain frequency there is a resonance buzzing which dies away when the fan proceeds on to full speed.
Try applying pressure to the rear of the unit near the exhaust port. My first GF (I am on #4, long story) had a similar issue, and it eventually started doing it all the time. I think something back there is glued and the glue comes loose on some units.
Here are the two threads on it. One of them has a link to a video so you can compare the sounds:
FWIW, GF ended up replacing the unit, i.e. there wasn’t some fix I could do myself. Hopefully it is a different issue for you, but I thought I would pass along the info.