I think you need to check you spark plugs. Is it possible you didn't get some torqued down enough? To get that noise and not have done any exhaust work it is the only logical solution. If you take the coil covers off you may even be able to feel the compression/air surge. Keep us posted

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600