RZT 42 clunking noise problem when machine is sitting at idle

[50missioncrush]

Just started up my mower today and it has a heavy clunking sound coming from underneath. Don't think it engine related but rather seems to be from the hydrostatic pump or transmission areas. Machine is old and has not had maintenance before I bought it a year ago. Has run with no problems after installing new belts and blades, no engine oil useage or smoking and may leak one or two drops of oil off bottom of PTO per week. Sound is evident with machine sitting still and frequency seems to be related to engine RPM. Seems to smooth out at full operating throttle?? Any suggestions as to where to start troubleshooting?

 

[ILENGINE]

I would look at the hydro drive belts, and tension idler pulley. Belt could have section missing or maybe a stick or rock stuck in one of the pulleys on the hydro unit,