I have a serious vibration through the steering wheel when engine is idling in drive. Also after driving for few a minutes steering becomes stiffer and there is a low humming noise, sounds like it's coming from the right hand side in the engine bay of my volvo S40. any ideas would be grateful.

This is very serious and I would advise not driving it AT ALL, until a cause can be identified. Although you said it's "not the front end wobble", it sure sounds like there still is something wrong in the front end and/or the alignment. It could be broken engine mounts, missing hardware/very loose control arms, steering box connections, loose steering column, transmission mounts, driveshaft bearings/mounts or even loose lug nuts on the wheels just to name a few. Please have the car safely raised on jacks and have a mechanic or knowledgeable person look underneath for broken/damaged/missing parts.

When I drive at high speeds I hear a funny siren sound with blue lights flashing :) Seriously though, the power steering fluid may be aerated or low and moaning which can make the steering wheel feel odd, as in vibrating . See what your fluid looks like and the level of it.

Hello,

If problem only occurs 'under power' sounds like a u-joint (driveline - rear wheel drive) or a cv-axle (front wheel drive). Check for excessive play, fluid, boot tear and breakage.

This is a rough idle, what is causing it is a big open question, you will need a tech to evaluate the problem. It could be just worn out engine mounts or a slow idle issue or many other things.

I would see if getting new tires that are goodyear, michelin, or Hankook solves the problem on the front. Tires can be tricky even when balanced and I wouldn't be surprised if that's the source of your problem. Fully inflate the tires, too.
Hope you can fix your problem, without too much trouble.

check engine and gear box mountings

if it was my i would be looking at the drive belt and the water pump give these websites a try www.autozone.com and www.alldatadiy.com if all fails stop by your local library and get your hands on a Haynes auto repair Manual for your car or stop by your lincoln dealership in the service department and ask for the manger but please do not keep trying to drive the car cause it will do damage to the engine nice car wish you the best of luck Michigan Man.

You have a shot front wheel bearing which will sound like a whining rumbling noise, if the bearing is really bad it will have started to break up inside and the streering wheel can snatch.

Guy with a trolley jack can tell you in a few minutes how bad it is

Vibrations while driving like you are describing can come from tires that have defects (internally broken belts) (wheel weights that have fallen off) or a bent rim (perhaps from another driver [not you :-) ] hitting a curb) Are the tires worn? Have they been balanced and rotated regularly? You report no vibration in the steering wheel.
Vibrations during braking can be warped rotors or discs (but this doesn't sound like your problem since these kind are usually felt in the steering wheel during braking).
If and only if you feel like you can do this safely,
you can try getting the car up to 55, placing the car in neutral, then switching the "engine off" but leave the "key on" (be warned the power steering will go out but the vehicle can still be steered though the steering wheel will be much stiffer) if you try this (do it perhaps in a vacant parking lot or on an empty road) and the vibration goes away with the engine off but while the vehicle is still moving, then the vibration is engine or transmission related. If this is the case you might hope for maybe a bad engine mount. Bad engine mounts can be diagnosed with the vehicle stopped as well. However, if the vibration remains with the van moving forward but with the engine off, the vibration is likely a problem with the tires or suspension. Try to provide some more info.
If you don't feel comfortable trying the above suggestion, please take the vehicle in to an ASE certified suspension and steering professional for further diagnosis.

Finally, I had a similar situation happen to me today. I was riding with a friend in their van and the vehicle started vibrating more and more around 55 mph. We stopped and carefully checked the front tires (including the lug nuts for tightness) but
found a tire defect. (it had several broken steel belts that caused a bulge in the tire and thus the vibration) We changed the tire and the vibration was gone. Good luck to you.

Engine vibration is the cause of this issue, it is the biggest complaint from many owners, changing the engine mounts will help, as mounts age the rubber gets hard and they don't absorb vibration like they should, new mounts help issue, but it will no stop it altogether. A faulty MAF will not cause vibration, just a hesitation on acceleration at times