I actually had and repaired a very similar problem in my truck radio a few months ago. If you want to pull it out and run it on the bench I may be able to help you. Mine was due to a bad 100Pf capacitor in the final detector stage. They are tiny capacitors which are normally not replaced in a rebuild. To speed the process up, I pulled tubes one by one from the first input stage until the noise stopped. The one frustrating thing that can happen is that you pull it out and it might work perfect again. If it were mine, I would run it for a while in the truck to make sure it will stay broken!
