if it was a pistio it would be a knock noise and not a rattle a valve noise  will very with the rpm if that is not the case  than check the heat sheilds that cover the exhaust system you can do that by just tapping on the pipes at diffrent points and see if it sounds the same if it does than most likly you have worn rivets that are holding the sheild plateing to gether, or you may have a cat convertor that is braking up or the stuffing is becoming loose and that can bechecked the same way, you may all have a problem with bad gas and the enging is pinning, you can use a octane bust to corret that  problem, good luck