Rattling/Fluttering Noise 2900 - 3100 Rpm

[pjq]

2004 Boxster S
Hi its been awhile since I posted here, I've been enjoying the car BUT like most of us I would like to have more time to be able to do more road trips.

Recap, because of an IMSB failure (71000 km/44000 mile) in Nov 2012, I rebuilt the motor between Jan - Sept 2013. Its been my daily driver between May and November and now has 104000 km/65,000 mile.

There has been this faint Rattling/Fluttering engine noise, sort of like a worn metal gate valve that is part way open with fluid running past it at high rates. This only happens between 2900-3100 RPM. I notice it when in 2nd or 3rd gear not so much in 4th. I try to run the engine at 3200 RPM and above. When I broke in the engine after rebuild, I did mainly highway speeds but also some city driving (always trying to keep the revs above 3200).

Other than this rattling noise, which may be a little more prominent now?, the car works good, especially when I'm by myself!
I run Joe Gibbs DT 40 and the filter and magnetic plug are clean every time I do an oil change.

Any suggestions where the noise might be coming from?