If there is a poor intermittent contact somewhere in the heating circuit of the rear-window defroster, it can cause radio noise, as the heating grid also acts as the radio antenna. (Unlike your situation, this problem is usually more severe when the heat is on and the bad spot is "sizzling" a bit, but turning it on momentarily might toast the bad spot and reduce its noise when turned back off and it cools and contracts.) Do all of your heating traces work well?

 

I don't know about the defroster being the problem. I remember on my 05 going through the defroster fix, the door lock fix, and a few more. All would work sometimes, none permanently. The brass strip repair has lasted me since 08.
I'm tempted to feel that Volvo has replaced the faulty part on newer cars but then again, how many little Volvo "idiosyncrasies" are passed down year after year.

 

P { margin-bottom: 0.21cm; } have a 2007 p2 v70 petrol 2.4. When I bought it I could not find any AM channels on the volvo hu-650 radio. I changed it for a clarion cz215e. The sound was excellent on AM and FM but AM reception disappeared when the engine was running. FM still OK with the engine running. I mentioned the above resistor fix to 2 independent volvo garages and a car radio specialist. All seemed to doubt it but as they had no other explanation I removed the panel above the window to expose the aerial amp, cut the power wire and powered it independently using my motorcycle battery. BINGO! Excellent reception on AM and FM with engine running or not so I am off to buy a resistor.


Another interim fix I found was to plug my very cheap Aldi Envivo portable DAB radio through the axillary input to the clarion radio. Took the output from the DAB head phone socket and powered it using the 5v USB phone charger adapter in the cigarette lighter socket. It is the best quality radio reception I have ever heard in a car. May depend on your local digital signal strength but with the little DAB telescopic aerial I had no problems receiving all the digital stations in the Greater Manchester area (UK). Could have kept the old volvo radio as that has an aux input hidden in the arm rest.