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Message: <blockquote data-quote="Tazza" data-source="post: 15652" data-attributes="member: 82">I have a "new to me" 753. It had injection problems which i managed to sort out after someone sabotaged it!. It also had a drive problem on the left side. Before i attacked the motors when running, the hydraulic pump would let out a hum which I'm sure isn't rite. I thought it may just be air so i pulled both drive motors and found on one side the seals were like powder. I replaced them and re-installed the motors today. Re-filled the tank, it drives great but that hum is still there. At low idle its not there but if you lift the arms and let them fall it hums as it drops. Increase the revs and it hums again. The only thing i can think of is the charge pressure relief is causing the noise, but not sure.Any ideas on what it could be? I had a 743 that would do this from time to time, but i never did find out what the noise was from.</blockquote>