2004 Z4 2.5i (M54B25) – Whistle noise at light throttle between 2000–3000 rpm

[Maximus_E85]

Hi everyone,

My E85 Z4 2.5i (M54B25) has a whistle/high-pitched noise that only occurs at around 1/6-1/4 throttle between roughly 2000-3000 rpm.

Some observations:

• No check engine light.- (before and after replacing the intake boot)
• Engine idles smoothly.
• No noticeable loss of power.
• Noise disappears completely with more throttle, less throttle, or outside that RPM range.

I recently replaced the intake boot because it had a pretty large crack in it (the small branch to the idle control valve was split open). After replacing the boot, the whistle became much quieter(90% of the of the original noice). Before, both driver and passenger could hear it. Now I can still hear it faintly, but passengers generally can't.

Since the noise was reduced so much after replacing the boot, I'm thinking there may still be a small vacuum leak somewhere, but I'm not sure.

Has anyone had a similar whistle on an M54 that only appeared at light throttle in this RPM range? Or has anyone faced any leak somewhere else that is highly the cause here?

Would you continue driving it as-is, or is there a specific area you'd check next?

Thanks!

 

[enuff_zed]

I'd start by checking that the boot is properly fitted. It is very easy to not quite get it fully seated on the throttle body, then when the jubilee clip is tightened it pulls it out of shape and opens a small gap.