ok, so spent the past hour or so troubleshooting this.
Before I turned the ignition on I did a test, no noise. I did a 4 channel balance/fader test just to be sure all was good there. I found that the signal into the front left speaker was quite weak compared to the front right. First thing I did was reverse the left + right speaker cable on the amp, the problem followed the cable to the right front speaker...problem in the speaker cable connection to the amp?! Unhooked & rehooked the cable several times, the problem was not with the connection on the amp side. So, I gave up on that problem & moved onto the bigger problem, the engine whine.
Starting with the earth, found a solid place for the cable and dremeled the paint clean off. Tested, nope..still loads of engine whine. BUT....I noticed it was mostly (or all) coming from the bum speaker I tried to fix earlier. Went to the amp and unhooked the speaker. Bingo, clean signal in the rest of the system.
So, the speaker cable is what's causing the problem & it seems it's only to one speaker. Now, as much as I hate to admit it, I think I'm probably a victim if my own lazyness....I may or may not have been as carfull as I should have been while hooking the new speaker cables to the OEM lines. When will I ever learn....
2 possibilites as see them are
1) as stated above, it's behind the head unit
2) the cable has become damaged somewhere between the amp & head unit.
Any other theories ?
I'm still confused as to why this did'nt seems to cause issues with my other amp?!?
thanks for help above.
Cheers