Noise in 520

..... .A Few Things To Check....

~Is the noise once per rev, or 1 /2 engine speed at a cam speed?
~Does shorting either cylinder change the noise?
~Does putting a load on the hydraulic pump change the noise?
~Does running with the PTO engaged or dis-engaged change the noise?
~Does putting a PTO load on it change the noise.
~Does running with the clutch dis-engaged change the noise?
~If power steering, does turning the steering wheel change the noise?
~Does retarding the ign timing change the noise?
~Do you get the noise when towing the tractor in gear with the ignition off? ~Do you get the noise when towing the tractor in gear with the ignition off & spark plugs out? ~Can you get a similar noise when grabbing the fan & rocking it within it's play?
~ How much crankshaft end play do you have?
~Try increasing the crankshaft end play & using the clutch lever to lightly load the crank play to the right --[if the crank thrust washer is worn the crank throw could be hitting the cam gear, this should show that].
~Drain the 1st reduction cover & put in extra & cold 140 weight & see if you get a noise change [it won't stay in there long so you will have about 1 run to evaluate the change].
~Pull the crankcase cover & run each piston to the rear & reach in with your hand & wedge your finger between the piston & rod side & have someone rock the flywheel & feel for loose piston pins.
~Make sure the flywheel isn't hitting the starter or cover.
~If you can post back the answers to some of the above maybe we can help you isolate the noise.