Alright so i won a auction on a 2007 Infinity fx35 104k miles, mint and very well taken care of. i have a decent scan tool and the only codes it has was catalytic converter code. Found out it was clogged and needed to be changed out. On the way from bringing it from the auction, i notice one issue, when in 1st gear if revved passed 4k the rpms would jump up and down. (similar to giving it gas and letting go really fast) still ultimately accelerating. I did some basic testing like putting it in second and acceleration on a hill to check the clutch, playing with the gears up shifting and down shifting with the manumatic tranny checking if it may be related to it... nope butter smooth. i changed all the fluids and everything seems fine but the cat issue. recently i changed the cat to some high flows which leads to another issue. With the HFC the car runs better and more lively but if i accelerate semi quickly mostly in first something on the drivers side make a grinding noise (imagine a bolt slowly coming into contact with a high spinning object like a metal gear or kinda like stripping gears in a rc car, you can hear a high frequency of grinding on something that isnt smoooth). If i accelerate at a moderate speed i dont get this noise at all, which it feels like something is leaning back and hitting something spinning. Found out if i accelerate hard enough in second i can get the sound but it sounds more like something is swinging into it or tapping (grrr. grrr.. grrr) then clears up because there isnt much more pull in the acceleration at high rpm. It did not occur with the original cats. The sound most occurs when there is a lot of g force in action. low gear with a lot of pull. everything shifts fine and smooth engine seems healthy. let me know of your idea on what it may be. I know the wheel bearings go bad on these but thats not the case. Perhaps new engine mounts, might be torquing too much and something ends up rubbing not sure. I checked underneath hoping to see something damaged but from what i can see .... nothing. Any input is good input i have a friend Mechanic but even if cost are reasonable i would rather do it myself since i am mechanically savvy. Thanks for you time reading this
Your first gear is going out.
But if I accelerate hard enough in second without letting it down shift and in third i can get the grinding noise. If in second It just grinds for a seconds like a tap and comes back until i pull out of the lower rpms. its even less prominent in third hard to make it happen. To me doesnt sound like a tranny issue. also saw that my engine mounts are shot and i get massive twist underneath. Perhaps something is rubbing?
You can check engine and transmission mounts and pray that the issue is there, not the transmission. If you put your car in park and rev up, do you feel the grinding?
I do not get any grinding in park.
Odds are that the transmission has some issues. Take the car to a transmission pro and have him check it out.