Hi again,
I have just jacked up the car and inspected the underside, nothing obvious.. I had removed the metal skid plate, and the fiberglass heat shield and went for a quick test drive, and the same noise was present.
I can also "Feel" the grinding when it kicks in, through the gas pedal and chassis feedback. Leads me think it could be drivetrain/ transmission related?
I'm thinking of taking it to a local 86 specialist this weekend, and seeing if they diagnose the cause of the noise.. But I'm not throwing in the towel yet! I'm eager to try anything else
Thanks for you help!