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Hello, this is my first post here, so please be gentle if my question is stupid

My Briggs and Stratton lawnmower sounds unhealthy (as in video below).

Thing I have already do/done:
1. Use petrol with stabilizer, same as always.
2. Cleaned air filter and box.
3. Removed carburettor, cleaned it with carb cleaner spray, replaced with new diaphragm.
4. Shortened the governor spring to up the revs (it was almost stopping at the low end of the revs before I did).
5. Cleaned spark plug
6. Replaced engine oil.

After all of that, it still sounds like seen in the video here:
Any ideas?

While it is running, tilt the mower back on its rear wheels, and see if it changes.

Also, take off the top again, and look at the kill wire going to the coil, see if it is close to metal and shorting out. Or just disconnect it and then run it to see if the problem is there, but you would need to yank off the coil wire {shock} to stop the engine.