How old are your ignition components? Plugs, wires, cap, rotor, etc? If they've got better than 5 years or 100,000kms on em, I'd replace the whole lot. That will help.
Timing belt, as stated above, is a biggie. These engines are interference designs, which means if the timing breaks, the valves are stuck out far enough to get the hell mashed out of them by the pistons. This basically requires an engine rebuild and new head to fix. Very costly. Also, I just did my timing belt and drive belts yesterday, and I can tell you that yes, it makes a performance difference. If your belt is stretched from age or wear, your timing essentially it retarded, and this bites into your power and other things. Mine was, thats why I know. If you're competent around cars, buy the Haynes manual, and then buy the belt (do the drive belts while you're at it) and you'll be a worry free driver for 5 years / 100,000kms, which is BMWs reccomended interval for belt change.
For the A/C, check the system out really well for wear or spots that might leak before recharging, or you're basically pissing your money away on refrigerant. Nobody likes to do that
ghead . If you can, get it converted to R134a. Much more environmentally friendly.
I'm jealous. I don't have A/C. And it's getting hot out.
After that, if the rumble persists, it could be anything from a motor mount to a timing issue or just age. When you want to refresh a car, do what you do with all cars. Give it all new fluids (don't forget the differential) and filters, do the ignition system, and the belts. You should be alright from there on in.
By the way, congrats on the Bimmer, and
. Will we see pics soon?