2008 M35 Knocking Noise Help

[2008M35]

Hello guys, new forum member here. I have a 2008 M35 with 51k miles and the only mod is a 5/16" plenum spacer.

Started about a month ago the engine developed a knocking noise that follows the RPM (the frequency increases as rev goes higher, and when it shifts into next gear it starts from low again). What makes this very strange is that the noise only comes on when engine is warmed up and under load, in both drive and reverse. No noise in park or neutral or coasting when in drive. No noise when engine is not warmed up.

I put in a fresh tank of premium gas and a new oil change, nothing helped. I drained and filled 6 qts of fresh Matic S 1 year ago, and the fluid still looks nice and pink. I am out of ideas of what this noise can be! Can you guys please help or chime in your $.02?

Thanks in advance!

 

[Freddie's M45]

If it doesn't knock all the time I would check the fuel pressure damper and pulley tensioner 1st.

2nd would be knock sensors which only happens when eng is warm to hot.

 

[2008M35]

If it doesn't knock all the time I would check the fuel pressure damper and pulley tensioner 1st.

2nd would be knock sensors which only happens when eng is warm to hot.

If the knock sensors detect anything, wouldn't it trigger the check engine light?

I will check the pulley tensioner, but I am not sure what is a fuel pressure damper. Is that the same as fuel pressure regulator?

 

[Freddie's M45]

Most of the time you don't get a check eng light with the knock sensors.

You could get the sound just by filling up your tank with cheap gas and driving on a hot day. The more load on the eng the more noise you'll get.

 

[Freddie's M45]

any luck finding the problem?