That noise you heard was the trans slipping/mis-shifting. You had no sport mode because the trans was in failsafe mode. Once you cycled the ignition, it went back to normal. You need a BMW GT1 to read the codes. it will probably show a mis-shift
The problem is most likely, the solenoids in the valve body are hanging up. It's a common problem that could of been exasperated by your fluid change.
If it keeps on happening, You could probably get away with replacing the valve body but if it keeps on slipping too much it will burn the clutches and you'll need a new trans.
We just went through this on our X5 a week or so ago... we had the same problem/noise start about a month or two ago. Eventually, it just got worse and worse. luckily we had a warranty that covered a remanfactured trans from BMW...