Agree odd only at the 1500 RPM and thus may not be a bearing. As KJM suggested a vibration of some sort that hits its natural frequency at only the 1500. Like a loose heat shield on a car exhaust > only comes at certain speeds. So, got me to thinking what else could do that?
Try the no belt method as first step BUT is that is not successful, then get yourself one of these
dual-device mechanic’s stethoscope (
link) that has the ‘sound cone’ as well as the rod (see pic). The cone allows you to rapidly localize the area the sound is coming from > simply wave it around different parts of the motor until you hear it the loudest. Its akin to the Geiger counters you see in movies, they wave it around listening for the spot that provides the most rapid-fire clicks and bingo, they found the isotope. Same idea with no need to touch the device to the motor like you do with the metal probe. With the spot more localized than look via the rod and/or start trying to wiggle anything in that area to see if loose. One possibly is the shield between the bell housing and the motor, see pic.
I know we all hope it’s something simple and not drastic like a timing chain about to give up :-(
