Taylorlashae

I've had this constant rattling in my car. I have a 2000 v6 Ford Mustang. It doesn't rattle idling I can sit and rev my rpms up to 7 and it not rattle, but when it's in drive when I get on the gas it'll rattle at about 40-50 or higher. I had the exhaust manifold fixed. Recently heard it could be valves but I just want opinions please.