Might be ping. My 'tour pings like that after being on the highway for a couple of hours. I pulled into a rest stop and stopped at a stop sign, hit the gas, the pinging sounded like a a bunch of loose nuts in a coffee can. After sitting a while it doesn't happen. I sometimes get it with "cheap" gas too.
Try a tank of high test or octane booster or regane. If it goes away its ping.
