Heavy Rattle after installing SMF

I’ve just installed a SMF, weighs about 18lbs on my 370z. I was just wondering if anyone has experienced similar to me. When driving around, at around 1500rpm in 3rd gear I get a heavy rattle that gets louder with increased throttle inputs. I’m currently breaking in a new clutch so I can’t push the car too hard.

The rattle is none existent in 1st at same RPM, 2nd it’s there but not crazy, and then it gets a lot worse in 3rd. I’ve never had an SMF before so I don’t know if this is normal or not but it was quite concerning the first time I heard it. Almost nothing noticeable at idle either tbh which is what I hear people complaining about.

Reality is, I’m never really in that RPM range anyways, I’m just wondering on theories for this? Maybe it’s just a weird resonance thing with the fly wheel, or maybe it’s something that needs looking into more.