Question about boom at 5,500 rpm in 2013 X3 28i

Hi
Just got a 2013 X3 28i. This is my first turbo engine (but 6th BMW).
Usually at about 5,500 rpm (but sometimes lower, 4,500-ish rpm), i.e. when stepping hard on the pedal, there seems to be a boom, like from the exhaust. It's kind of a low-pitched sound, which is hard to position in space but sounds like it's coming from the rear of the vehicle. It is not an unpleasant sound, actually kind of cool - sounds and feels like a rocket booster firing or something. I've replicated it three or four times; it occurs with the pedal floored once the RPM hit above 5,000, and last just a second. Can anyone identify what this is? I'm assuming it's something that occurs with a turbo engine. Either that or something is tremendously wrong with my X3 LOL
Thanks

Just curious how many miles on it, since that RPM is beyond break-in recommendations. Are there any flames shooting out of the exhaust and do you enter hyperspace?

Originally Posted by noka

Just curious how many miles on it, since that RPM is beyond break-in recommendations. Are there any flames shooting out of the exhaust and do you enter hyperspace?

Busted. Boom and bust.

I was going to wait for 1200 before trying anything hard, but I had about 1050 miles when I first tried flooring it. Am now right about 1,200 miles. Have only done it 4 times. It does feel like I'm entering hyperspace when I do it. It has a vaguely science-fiction-y feel to it, a low boom/roar for just a second right when the RPM peaks at about 5,500 and just before the gear shift.

My Audi does this too. 2.0t

My F25 35i as well booms -- I thought it to be turbo-related but not certain.

For me the boom is always timed with a shift. I think it is just the loud, quick shifting on that kind of tranny when you are in Sport mode.

The boom is the exhaust dampener opening at 5K rpm then closing again as the rpm dips back down. In normal operation exhaust only comes out of the left tail pipe. Above 5k rpm a dampener on the right pipe opens to allow for the added exhaust flow. You can see the dampner mechanicals if you get down on your hands and knees and look up at the tail pipes after they connect to the the muffler.
I was following a 335i the other day and in our cold climate you could see exhaust coming out of the left tail pipe while the right one was full of snow.