I'm trying to remember, we had a 48 that did this. It was a very low hr. machine. I think the large spring on the tensioner was weak. I will ask on monday when everyone comes in. I just can't remember.
My full size tracers do this once in a while too, always when engaging the blade at low rpms with a cold engine. I think the belt may be stretched a bit coupled with the whiplash the idler gets from the engine bogging down when engaging the blades.