Cart has thump, thump etc pedal up

[Pumper]

It is a 2004DS IQ, while going forward under pedal down all is OK. Release the pedal then I hear the thump, thump.......thump. The frequency is related to speed and it happens while traveling straight.

I am suspecting the motor (it is a D&D high speed motor controlled by Alltrax DCX 500 amp controller), like to hear your thoughts before I try another motor, I have a Plum Quick Purple motor I could try.
I have not ruled out the differential but it is not caused by turning.

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you can make video of cart and then post it so we can hear noise

[Pumper]

Currently I don't have a way to make the video, no I don't have a smart phone.
I will try to find someone with one.

I put the Purple motor on and it still makes the thump, thump....thump.

With the motor off I took the tire off the driver side and rotated the drum every two rotations it would bind slightly just needed a small increase in force then it would repeat. Other tire is on the ground so it would not rotate, if that is related to the thump then I suppose it points to the differential.

It is a Graziano axle with 8:1 gears.

[theboss20]

Jack it up and put it on jack stands...use a piece of an old broom handle (wood) get somebody to engage the motor and spin the wheels while you listen in different places to determine where the noise is coming from. A mechanics stethoscope works best but not everybody has one. If this test shows no issues then suspect the tires...could be a broken belt in the tire. Or rotate the tires front to rear and see if the thump follows...let us know what you find.

[Pumper]

That's a great idea, my wife was a nurse and has a stethoscope so I used it.

It appears the loudest noise is coming from the differential area. Not being an expert at this so I might be wrong but I guess I need to tear apart the axle and look in the diff area.

I am quite sure the noise id not coming from the motor or the axle shaft area.

Doing this test also told me the thump, thump etc is not only on decel also while the motor is in steady state.

[Pumper]

I think I found the problem but won't know until new parts are installed.
What I had not mentioned was I had installed high speed gears which included all three sets of gears.

I found by inspecting each gear tooth that two teeth on the ring gear had been damaged apparently in transit to me. The damaged made a bulge on the tooth face, not large but I believe enough to cause the thump.

I will inspect every tooth on the new gears when I get them so this won't happen again.