I had the same problem once. I upgraded my amp to something bigger and got the whine. With my old alpine amp (which i still currently have) there is no whine, i tried all sorts to fix it, different ground points for the amp, those power supressor things, no luck. took it back in the end.
The suggestion of the ground fromt head unit is a good one, i'll have to check that, i'm pretty sure its a proper ground earth, but i'll double check to be sure next time my dash is out.
Ohh and to the OP, check your spark plugs have the R in, eg, BKR6E.