Had brake pads replaced due to squeeks and now its happening again after car is driving for about 15 mins.My machanic whom i trust said he used very good quality pads so any ideas anybody.I have also noticed a high pitched noise coming from engine intermitintly and an annoying squeak when i drive over bumps which there are alot of on Boston roads.I had front wheel struts replaced early this year.
Thanks folks.

Ask in Roadfly forum. There are more E31 owners there and are very knowledgable, I get all my questions answer there. Give it a try,

http://bimmer.roadfly.com/bmw/forums/e31/

There could be a multitude of things. First the Brake Pads WILL squeek. The more dust a pad generate the softer the pads are and less they squeal. Also for BMW, every brake pad has a very thin paper like shim between the pad and the caliper. Those needed to be transferred to the new pads and use of Anti-Squeal paste will reduce the squeal.

As for the engine sound..I can't really help without listening to the sound but chances are the engine mounts are on their way out. Also for the noise over bumps...the dust shield for your brakes may be loose or cracked.

HTH