rapping soud

I just replaced the bypass hose on a 1996 marquis with a 4.6 I found it necessary to remove the intake manifold since the hose/pipe runs underneath it. Thats no problem I have done about 5 of these manifold jobs. after putting it together and starting it up there is a rapping(knock) sound comming from the engine. Now keep this in mind there was no noise like this and 3 hrs later its there. I have checked for oil pressure and its all under mfg. specs does any one have any ideas??