exaust resonance
had same problem on my 68 RR i think their exhaust is tuned for headers, i've talked to a lot of guys with the same problem, they all had stock hp manifolds, i installed a pair of magnaflo resonators right before the tips. i got them at performancepeddler.com on ebay $50 each, summit wanted $64. http://www.performancepeddler.com/default.asp
had same problem on my 68 RR i think their exhaust is tuned for headers, i've talked to a lot of guys with the same problem, they all had stock hp manifolds, i installed a pair of magnaflo resonators right before the tips. i got them at performancepeddler.com on ebay $50 each, summit wanted $64. http://www.performancepeddler.com/default.asp