Humming 3,0k - 3,5k rpm

Hi, Since I have the 740 turbo '87 B200ET I have noticed a humming noise at medium revving, for instance highway (most of my km have been in). It is not uncomfortable, but we need to loud up the radio...and conversation. Happens at all gear...for the last 35thou km... Knowing that the redblocks have been later upgraded with twin balancer shaft to reduce vibration, my question is: Being 700 series a good swedish roller, is this natural in these 8v engines? Or should I expect something is not right with engine/gbox? 2nd question, and having almost nothing to do with the previous: is the 16v engine block B234/B204 a real iron red-block or changed to aluminium? Thanks in advance

Vibration 

Yes the B234 is iron block - a two-man lift! I also have a resonance in my 760 turbo at 3,100 rpm worse on the overrun, and it's a damn nuisance since it starts at about 78 mph in top which is normal motorway cruising speed and I have to go tooo slow or tooo fast for teh 70 speed limit here in the UK. I also have a resonance on my 745 16v from about 5,000 rpm to about 6,000 rpm since I had to put a crap exhaust in in a hurry andI suspect a missing rubber about by the driver's feet. Input on these would be helpful.

Where is this humming coming from? From the engine? The Transmission? The rear end? It could even be tyres or bearings.

Hi MTerry I feel it is coming from the engine bay, and is rev dependent. I believe if it was a wheel or a final transmission bearing, couldn't stop it with clutch or neutral disengage... Having 30kmiles since the timing belt change, I can only make a guess on that system condition. But I own the car for the last 20kmiles, and was the same since I bought it. No change... Now I understand why Volvo put 2 balancer shafts in the last red-blocks...

Hi, Mister T Next shop visit (AC fluid re-charging) I will check the engine and transmission mounts for condition. My car has gone twice to official service, and nothing was said about it, I believe these part are checked in periodic maintenance...but it was some kmiles ago... Regards

Engine mounts eh? 

Ah, must try that. I am due new exhaust soon (was thinking of maybe SPeedwell or Powerflo or something) and could do that at same time...