No, nothing like that you dirty-minded people...!
I have a 2009 135i (N54) and I've heard that it's "normal" for our cars to occasionally "wobble" or "throb" a bit when idling - felt as a small jolt through the chassis when sat idling in traffic.
It's never really bothered me, but last weekend I changed my transmission mounts for the M3 items and didn't notice anything different all week. Not sure if the car wobbled at all, but I certainly didn't notice anything out of the usual.
Then this weekend just gone I had new spark plugs fitted (BMW-branded ones from the dealer). Car seemed fine on the way home, then I didn't drive it Sunday. I also installed a new panel filter (just an OEM-style paper replacement from Bosch).
This morning it was cold, approx 1 degree (centigrade) above freezing. The car seemed a little hesitant in its idle when I first started it, though I did have the heated rear screen, heated seat, and max blowers on the windscreen all going, so I'm not sure if it had something to do with that.
Anyway, then I drove a while and when sat idling in traffic felt a really quite violent throb - almost felt like the car behind had just tapped my bumper.
Then a minute or so later it did it again. I had the radio off at this point to listen out for anything, and couldn't hear anything, though my windows were up.
Any ideas what this could be? I'm worried it's something to do with the plugs, but thinking that maybe I could just be feeling the wobbles more because of the stiffened transmission mounts? Also seems weird that I've not noticed any wobbles in a whole week, then got two within about a minute this morning. Hoping it's not a missfire or anything like that - though I have no idea how to diagnose.
No warnings or fault symbols have popped up on the dash at all.
Thanks in advance.