Chattering noise.. search narrowed down.

I've recently experienced a chattering, almost pinking noise coming from the passenger side rocker cover. This happens only when the car is under load accelerating or kickdown etc.

Not being all that technical the car (340 Demon) was taken off to a rolling road where pinking/detonation was diagnosed not before trying to jump off the rollers and spit them out several times. The timing was backed of 5 degrees in total. Guess what, drove off and nothing had changed.

The thing is this noise is as bad when the car is cold as when it is hot and it is not affected by the weather etc when pinking usually is.

I did a few runs today on a quite road pulling each plug wire in turn until the noise actually went when I pulled the second wire back from the front of the car passenger side. It also made no noise when this was re-connected and the third wire back was pulled??

It seems when the load was taken off either of these two cylinders the noise stopped. I've now sort of isolated the noise and just need to know your thoughts on what it could be. Suggestions please Gentleman..

Oh and yes the timing will have it's 5 degrees put back on once sorted..