I have been noticing a weird rattling noise from my 04 (still non-turbo) around 2500rpm while under load, I had the dealer check for a lose heat shield and they said everything was in good shape. Could it just be the header or down pipe rattling around? Has anyone else noticed this on their miata?

I run 91 octane on the car as recommended, the car is basically stock besides tein springs and a K&N air filter, lol.

I'm just wondering wtf is going on with my car and if I should be concerned, it would be pretty pathetic if the OEM computer can't run this thing with out knock. Thinking about installing the MS sooner than planned if the OEM computer sucks this bad.

 

The stock ECU has a knock sensor. When it senses knock it retards timing accordingly so you should be fine really.
Knock sounds like a bag of marbles being poured on your motor sometimes, or something kinda like that.

 

If you've got a stock '04 and haven't touched anything, then I seriously doubt it's knock. The dealer should be able to pull codes on the ECU to see if there were any knock events (assuming they would've done this if you brought it in suspecting knock).

Easy way to tell would be to get ahold of some race gas or toluene and mix it in with your 91 octane and see if the problem goes away.

BTW, usually knock is going to occur around torque peak (~4000-4500 RPM on our cars), so I'd find it extremely odd if it was happening at 2500 RPM. The knock I've heard tends to sound more like a buzzing sound than a rattling sound...actually, I've heard a more rattly-sounding knock on my parents' Greddy'd car before, too. But again, it's always happened around torque peak, as that's the most likely time for it to occur.

 

there are different types of sounds.

one is the buzzy rattle but it can also be at high load, low rpm (just taking off, almost lugging the engine) and it'll sound like a more distinct few ticks or pings coming from the inside of your motor. like tapping a ball peen hammer on an anvil.