my to-35 started making that type noise, coming from the front of the engine area.
the big nut/bolt that holds the front crankshaft pulley on was loose,
we cleaned the threads then added a light coat of lock tight.
then as others said, tighten it with all we had using cheater extension.
motor ran fine, just like a fergy sewing machine.
I was sweating over the noise, thinking the motor was going to have to be rebuilt.
every once in a while, even a blind hog finds an acorn.