I haven't looked At anything yet, But it's pretty weird. The machine makes a random noise that Actually thumps the welding cart. The noise coincides with a puddle or weld hiccup. It's very random.
If it's a wire machine, check your trigger or your liner. (short in the trigger or worn out liner) Had same type issues with my mig once. Those are the only parts I've ever had to replace.
Mine acted like that one time. Replaced the lines and the gun. Still did it. Was just about to take it to the shop as that 220v electric scares me, especially with transformers and stuff. Took the cover off for a look see and found a loose connection. Simply a mechanical problem.
Like Deuce Daddy Don said the spool reel can cause issues if it wire feed. I had a similar issue and it turned out to be the roll of wire causing the problem. Even though the spool was correctly set in the machine. The wire was wound weird of the plastic spool from the factory kind of over lapped.
My old NAPA welder acted like that and I tried all the adjustments and nothing helped,I took it to napa to have it checked out and it turned out to have a bad circuit board,that is no longer available but could be retrofitted,the cost of the repair was prohibitive so I trashed it and bought a Miller. HRP
My Miller 180SD tig with digital readout was shutting itself off and throwing a general trouble code. Ground cable was loose.
mine did this and it was a crimped gun hose , the liner was crimped ( by my buddy who borrowed it ) and causing the wire to bind . had to buy a new hose liner . ( $20 ) it was causing the pinch rollers to jam up and banged when the tension released .
I've got a 1974 Miller 35S that still works fine, but found that some wire spools are wound badly and it'll get to one of those spots and do what you're describing. Several wire suppliers advertise that their wire is flat wound and that's what I'm currently using -seems to work fine. Of course keeping tight contact tips in it helps a lot.
Thumping noise makes me think it is the contactor dropping out; maybe bad connection at the trigger switch, bad trigger wire in gun lead, bad connection at the cabinet, wiring to or connections at the relay, wiring from relay to contactor, or connections at the contactor. Have also seen contactors that have vibrated partially apart that would work intermittently.
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