I've been getting an odd sort of screech/howl on my '91 740 NA auto, with no difference depending on RPM, steering, or braking. It starts and stops at random and often with deceleration for few tens of seconds at a time, but goes away completely after 10 or so minutes of driving. It sounds almost exactly like the hum from rubbing the wet lip of a wine glass, except extremely loud with varying pitch. Sort of like a German Stuka dive bomber siren, but I somewhat doubt I've got one of those strapped to the frame. The car has between 300 and 400k on it, and sat for a few years until recently, so there's a wide pool of possible causes to choose from. I was thinking of either the front brakes or a driveshaft bearing. The calipers are noticeably seized on the left front wheel, but I don't think it would be enough to cause the noise, and it persists despite brake use. I've ruled out belts, accessories, or a vacuum line, since it doesn't appear during stationary engine revving. I'm hoping it isn't the rear diff, but I really think it's coming from the front end, possibly left side but that may be the sound traveling. If anyone has an idea of what this could be, I'd love to hear it. It needs a parking brake cable, calipers, and a new kickdown cable (I hope to God) so I'm weighing my options.

I have only recently gotten this car running, so it may be one of those things that works itself out with time. I figured I'd ask here before I bet on that though, in case it's a serious issue. Thanks.

 

I'd look at the brakes first, sounds brake related.

 

Thanks, new pads and calipers are on the way so hopefully that fixes it. They definitely rub hard, it would make sense.