Low idle and a whine

[Alfashark]

Brought the Coupe up to the top of the drive for a quick pre-warrant once over, all was well - running and idling smoothly. Brushed against the oil pressure gauge wires and all of a sudden, its down to a 400rpm idle
Still revs sweetly, and seems to have normal throttle response but it just returns to a 400rpm idle, and a very smooth 400rpm at that.

Checked out the tailpipe and it has a smooth note, and isn't blowing carbon or any smoke or steam... has got me buggered, neither Fabian (our flatmate) or myself touched any part of the engine apart from that sender wire...

Also, the engine has always had a whine that rises and falls with throttle input... We've pulled the front over off and used screwdrivers and stethoscopes on it, but have never been able to find the source.
Today this has gotten louder (as in blown small-block), we've just pulled the front cover off the engine and used the screwdriver stethoscope trick, and we are both pretty sure it's not the cambelt tensioner, nor the p/s pump... possibly the water pump or oil pump drive (I'm assuming the oil pump drive is at the front of a 5-pot?)

Any ideas as to where to start hunting or diagnosing this further?