Hi Alex, I had the same problem on my last Omega and I'm sorry to say I never got to the bottom of the problem even after trying everything that has been suggested on here.
In the end I put a piece of thick foam behind the rear seat and that muffled it a bit. After that I just turned the radio up!

I would say live with it if you can as I spent a fair bit of money and time on it until a very nice chappie on the M56 went into the back of me doing 90mph (I was doing 70!)
He was eating a sandwich and trying to change a CD at the same time but it solved my problem!