Open the speaker cabinet
Remove the speaker
With your fingers, move the speaker cone in and out, like a piston.
DOES the speaker make a scraping noise, when you move the cone in/out?
If so, the speaker is blown.
The speaker cone must be FREE to move in / out without interference (scraping) against the magnet.
This is a very common problem: speaker blown.
You can replace the speaker, or have it RE-coned.
No, it has nothing to do w/ speaker cable or EMI, etc...It's a mechanical problem...voice coil is scraping against the magnet.