Ours (9hp) was on the belt guard. We tracked it down and initially wedged cardboard in there short term. That worked for a while then we ground the offending edge away. That worked but chased the noise (less though) to another point. We wanted to screw the guard down with a sheet metal screw but there was no lip we could attach it to. Fortunately our testing ended before cracks on the guard developed.
It was something we made sure to take into account when we developed ours.
Drew Coates, AKA Mr. Force