Two days ago I experienced SMG failure with my 2004 545i, with 46,000 km (29K miles). Just started the car, and idrive warning shows Transmission fault, Failsafe program activated, R, N, 1st to 3rd available. OK, the dealer is about 5 miles away...I made it about 2 miles, had to stop briefly, and then could not get back into gear...stuck in Neutral. Had to get tow truck to dealer. Now dealer is telling me the SMG hydraulic unit (pump?) needs to replaced. Part is $5800 (Canadian) plus $1300 labor=$7100. OUCH!
I have extended warranty, so this should be covered, just waiting for claims adjuster to approve. I've only owned car 1 year, during my ownership, I have noticed the "purring" sound of the SMG pump run about 6 seconds every time I open the car, but also the pump kicks on about every half hour, while the car is parked. I have no idea why the system would continually prime itself while the car is parked...maybe it's a system fault and the root cause of pump failure?

 

i just bought my SMG, and it too makes the 'purring' sound just as you described it.

if you hear anything from the dealer about that being abnormal, please let me know.

 

The way you describe, you seem to have a 3rd party warranty. Please let us know if the 3rd party covers smg pump/smg tranny.
What I heard from some of 3rd party warranties is that it's a heck of hassle to get them covered smg stuff(I heard this from m3 forums) since
it's not manual or automatic parts.

 

That noise your hearing I believe is actually the fuel pump priming the system. BMW's without SMG make the same sound.

I was going to say if your out of warranty head to a indy. mech. and they should beable to fix that for around $1500 US.