Here are the symptoms my 5401 (1999 with 174K Miles) has just started entertaining me with lately. In this case, the car has a rumble emanating from what seems to be the rear passenger side to the driver, but of course sounds more like rear driver side to the front seat passenger. Low pitched rumble but high frequency, like those warning strips on some highways when you run on the shoulder. Actually one of my ride-along ace mechanics thinks its funny that it makes the same exact noise his son makes blowing through his lips to make airplane/car "motorboat" noises when at play. To me, not so funny. So far, BMW dealers have said, basically, "Gimme $6,000.00 for a new trans and the noise will go away, most likely." Two indies have said, "Gimme $4,000.00 for a rebuilt tranny and the noise will PROBABLY go away" yet another expert has said "Give us $750.00 to put in a new control valve assembly and it's 50% sure the noise will go away" , and that same helpful soul has also said "If its not the control valve assembly, then you need a new torque converter for only another $400.00 from us, and that should fix it, but we'll have to charge you $500.00 to install it." I'm sensing a pattern here, and not a very good one. Problem I have with all the assembled experts is that not one of them has any difficulty prescribing these $400.00 to $6,000.00 fixes, but when I asked each one directly if they would do the work and guarantee the problem got solved or refund my money, each and every one said, in a word, "No Way!" This after my paying them for the exams, of course. Meanwhile, my treasured 540 goes along making a "motorboat" noise somewhere in the rear that reverberates all through the inside of the car for 30 seconds of every few minutes I drive it at 45 to 55 mph in either Drive, 4th, or 3rd gears, which is of course driving me crazy and causing any passengers to wonder what is wrong with the car for doing this and me for driving it. When this noise/vibration occurs, the engine drops a 100 rpm or so, as if under added load, and the condition will stop immediately if you accelerate or slow down just a bit. Of course once you return to the dreaded 45-55 mph speed range, the whole cycle repeats again, and again, and again, until you stop and get out of the car in order to shout at it. Anyone out there had this happen with your cars, and if so can you tell me what's really going on and what will ACTUALLY fix it? Needless to say, I'm inclined to try the less ghastly expensive fixes first, but if the incremental fixes don't get the job dine, I get to spend LOTS of cash and still keep the vibration, while the lady of the house gets to let me know just how stunningly bright I was not to go for the big bucks first and be done with it, PROBABLY. Any help, anyone?