2000 Gmc Sierra Emission Vapor Canister Purge Valve

I have a 2000 GMC Sierra with a 4.8L. The vapor canister purge valve makes a loud thudding sound after driving for some time. You can only hear it at idle speed. It didn't start doing it until a couple years back after I did an intake manifold gasket job on it. Is it supposed to make that noise? They are fairly inexpensive to replace. Thanks to all