If the noise are present over the complete frequency band it doesn't sound like a ground loop problem (correct me if I'm wrong).
If you have a grounding problem it's very common that the pitch change with the engine rpm.
There are several threads about the same issue but I haven't read one with a solution.
I don't know if spectrum analyzer for a smart phone could be used to find out the fourier transform of the noise.
I think I will try that to know if it's white noise.