Engine Acceleration reduces with a mild hissing sound from engine bay

[farhanbd]

Hi all! This is my first post, probably on any blog ever.

Anyway I have a 2003 E53 BMW X5 4.6is. It has about 85,000 km on the odometer bt has had a brand new engine and a brand new gearbox installed only about 15,000 km ago (although about 10 years ago).

Since about 9,000 km ago, I have been noticing a phenomenon wherein the (usually great) acceleration of the engine abnormally experiences hesitation when the engine is within the RPM band of 3000 to 4000 RPM. After 4000 RPM the acceleration kind of restores itself to the usual level; however when the acceleration restores, it does so with the vehicle experiencing an abnormal sudden push (or a forward leap or a sudden step-up in acceleration). This sudden push forward feels as if the vehicle suddenly got released from something which was holding it back.

Also, about the time when the acceleration is lessened within the usual RPM band of 3000 to 4000 RPM, I can hear a 'hissss' coming from the engine bay. The sound is almost like that of an air leak.

This phenomenon initially took place in about every two or less out of 4 acceleration attempts. Now this phenomenon takes place in every 3 or more out of 4 acceleration attempts. Also, nowadays it seems to me that the overall acceleration , even in the other RPM band outside of the range of 3000 to 4000 RPM, is getting hindered. However it is to be noted that the acceleration deterioration in the other RPM band (outside of the range of 3000 to 4000 RPM) is more subtle than that within the range of the 3000 to 4000 RPM.

I will really appreciate some opinions on the matter.