[Frenchy]

Since you say the noise disappears with the rocker cover off, and the cover has contact marks from the rocker, why not try 3 gaskets to increase the clearance? As I don't own a Bristol yet, I'm not familiar with the Chryser type of rocker arm mount - is it a shaft, or a ball stud that it pivots on? Reason I ask is, with a ball pivot, the rocker might go into a resonant vibration mode where it not only pivots front to rear, but also side to side. And maybe this happens at speeds below 1200 rpm, but stabilizes around 1200.
If you had access to a shop with a strobotach, they could synchronize the strobe flash to slow down and slow-motion the movements of the rocker, and you might see some unusual motion not visible to the eye alone. I once solved a lost motion mystery on a reciprocating experimental lab test with one of these instruments - it's just a variable frequency strobe combined with a tachometer so you know how fast it cycles, and can match or vary slightly its flash rate vs the rate of motion of an object. This might show the rocker "dancing" in an odd way which could vary its distance from the rocker cover.