408 Question

I have got the car up and running all leaks fixed ready for the road... kinda.
I was taking it out for a spin to see how it did on the road too. Here is the question. I have a wicked vibration just a few RPMs off idle. Like rattle your teeth bad. But it you go up in RPMs about another 100 or so, it goes away. Timing is good. Probable not the best gas in it but all i had at time.

Any thoughts? Thinking it might be a vac. leak but not for sure

It a 408 with comp cams xe274h, stock 360 heads that are port matched, eddy air gap intake, tti headers, eddy 750 carb.