[akiriik]

i was just wondering is this startup sound perfectly normal for bmw?
(Exhaust flap is forced open all the time)

5sec
At first it sounds like the exhaust is leaking (doesn't come through the video very well)

30sec that sound is quieter

55sec car is alot quieter

Video:

[petripaa]

It is perfectly normal.

The car is heating up catalytic converter and retards ignition heavily (to let through unburned fuel to CAT). Once the heating cycle gets completed car gets silent. It is bit counterintuitive to dump unburned fuel to CAT to get the car emissions compliant as fast as possible, but that's how it is done.

This is happening with all modern BMWs and is very annoying if you happen to have freeflowing downpipe and exhaust. Most engine tunes allow one to disable the "coldstart", which disables this feature.