I had a very similar problem with an sstran transmitter which came on suddenly, presumably due to a fault with a neighbour's SMPS. It made it more or less unusable.
I greatly improved it by putting a class X capacitor inside a 13A plug across the live and neutral terminals. It might be worth trying that. Try a couple in various positions in the house. I also have similar interference on LW but the plugs do not help that for some reason.
I found the solution by chance when I switched on a lamp with a CFL bulb near the set. Here is a youtube video I made at the time
https://www.youtube.com/watch?v=pAuHIr7xbn4