I just replaced my spark plugs in my E53 X5 (4.4i). When I started up the engine it was running well, as always. With the hood open, I did notice a brief suction sound when I depress the accelerator, just before the engine revs up. When I drive it I do not detect any performance issues accelerating; in any case it drives like it did before I replaced the plugs. The engine has 160k on it and the plugs had never been serviced. So, I assume it has nothing to do with the spark plug service.

I may have had the suction problem all the time, however I never heard it: with the hood closed it is in-audible. Playing with it for a bit, I did notice that when idling, the engine sometimes hesitates revving up during the suction sound. I have a feeling I have an air intake leak somewhere? Nothing obvious when I inspect the intake parts as far as I can see them. Does anybody have a clue or any experience with this effect?

In the past I have also heard a high pitch whistling sound coming from under the hood somewhere. But I have not been able to pin it down. It only happens at higher revs and it is not always there. I have not detected it for a few months; only during the summer but that may be simple coincidence. It may have something to do with it?