Check the connections for cold solder joints, if they wires are soldered to the speaker. Or, the solder on the push on terminal. It should be shiny, not dull looking. If you checked the push on terminal and it felt tight, how about the wire going to the back of the push on terminal. It could have been improperly attached (bad solder) or badly crimped. Wiggle it, see what happens. Also check the back of the radio connections.
I once had a vehicle that was producing whining and crackling sounds. Pulled the alternator apart and found four diodes with one leg of one diode not soldered at the factory. Any loose electrical wires can be an antenna transmitting a frequence that the speaker(s) are picking up. Check all electrical connections - everywhere.
As **** stated, check the speaker cone. It should not be dented in. If it is then try putting a vacuum on it and suck the dent out. A vacuum cleaner with flow control at the nozzle and a modified piece of cardboard shaped in the form of a cone and as wide at the far end as the speaker should do for pulling the dent out.
That's a start.
Edited: if the noise is from an electrical source in the MH or engine, an L or PI filter consisting of a small transformer and a couple capacitors can be put on the input power line to the radio to filter the noise out. Shady-ol-rack used to sell the kits.