Yup, had the same issue - crackle on the line that disappears when the modem turned off. Noted the comment re noise from LAN connected equipment - but if the router modem can sort that out, it's a poor job.
Noted the 'tests' carried out to try to fault-find. The only one not listed that I've done is to simplify the circuit by removing 'surplus' kit - like telephones. So, from BTO supplied master socket, connect ISP supplied filter, ISP supplied lead, ISP supplied modem router and wifi to computer. That way, the whole setup is service provider supplied and they can't blame you.
With this setup, I still had DSL failures...
...so sent the ISP a letter - LOL
J/K !
Mike.
