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?

[The Radium King]

only when moving? can you get it to make the sound when out of gear? when in gear but clutch in?

[RandallNeighbour]

You know, it could very well be an aluminum heat shield and it's vibrating around that RPM range. I had a couple of elusive buzzing sounds and rattles and I couldn't figure out where they were coming from since it didn't sound like aluminum heat shield rattling and buzzing, but that's what it was. 5 minutes under the car with a hammer and I was much happier.

[pjq]

Yes, so far its only when moving but thats only because I never thought about trying it while stopped or out of gear. So I'll see what happens there but I think its got something to do with a load on the engine?
Aluminum heat shields and a hammer...... wow now thats putting something in its place!haaaa! i was thinking about tightening with a socket wrench! So I'll have to look at the heat shields also.
What about chain ramps, can they go slack and need adjustment? If so what indication would there be that they need to be adjusted?

[MWS]

Aluminum heat shields and a hammer...... wow now thats putting something in its place!

I assumed he meant tapping to identify rattle. It's amazing what sounds even a small stone can make. If nothing else, it's a cheap piece of mind, and might just save time looking for engine gremlins.

Best of luck.

[The Radium King]

Originally Posted by pjq

Yes, so far its only when moving but thats only because I never thought about trying it while stopped or out of gear. So I'll see what happens there but I think its got something to do with a load on the engine?
Aluminum heat shields and a hammer...... wow now thats putting something in its place!haaaa! i was thinking about tightening with a socket wrench! So I'll have to look at the heat shields also.
What about chain ramps, can they go slack and need adjustment? If so what indication would there be that they need to be adjusted?

i think you’ve made some assumptions that you need to go back and confirm before you start worrying about chain ramps or hitting things with a hammer. it could be tied to aerodynamics, vibration, transmission, engine, suspension, tires, you name it.

[jakeru]

It can be hard to diagnose rattling sounds which only occur under specific conditions! Honestly, it's hard to picture the sound based on your description. But, I would throw out there, a rattling power steering pump!
Rattly power steering pump.

I've had what I believe to be this rattle on my 2001 2.7 for a while, and it has a "fluttering" characteristic to it. It's been really hard to capture the sound on a recording, by the way, because it's most pronounced under certain load range, certain rpm areas (and I've noticed it will "come and go" about 3-4 times while accelerating from 2000-3000 rpm, so definitely some harmonic action), and also is most audible only when driving the car next to a sound reflecting wall (e.g., concrete median or wall) on the passenger side. Oh and by the way, best way to describe it is sounding like a metallic rattling noise. So, if your sounds seems to match any or all of those characteristics, check out if your power steering pump has any rotational slop or play in it. I'm pretty excited to have finally figured it out in my car - went through a lot of tests to finally get at it. Best of luck!

[tommy583]

I have found this car to be able to make so many weird noises from the strangest things. Worn motor mount makes strange noises around 3,000 rpm. Loose spark plugs have been known to make chirping like sounds under load. Exhaust leaks and or loose heat shields are definitely another source of many strange noises. It could even be something like a pulley, water pump, power steering pump or alternator. A sound clip would probably helpful for us to try and diagnose.