[bamabaxter]

Do you have vac advance hooked up? When you get up to speed and start letting off of the throttle, this is when vacuum starts to rise due to engine rpm and the butterflies closing. Your vac advance could be pulling too much timing and causing detonation. If that is the only time that it makes that noise, when you reach cruising speed, it seems like a good place to start. Only way to really check the amount of advance from the vac canister is with a vac pump.