I have a 2000 S40 with 94k. Usual maintenance stuff never anything big. I did just replace the FPR because of a slow start and now the engine runs like its new (almost). I usually just drive it in town to work and back. I took it on the highway and > 60MPH there's a thumping sound and some steering wheel shake. Only happens when accelerating. It's also happening when power is needed like going up a hill, etc.. Sounds like its coming from the axle/suspension area on the left side. Any thoughts?

 

Could be several things unfortunately. None overly serious especially if you do the work yourself. The first thing that came to mind was motor mounts. I've replaced all 4 on my 2001 over time as each cracked with age. The 2 that would allow the engine to rock back and forth under acceleration/deceleration are the front and rear. These are not the big ones on each end but are the 2 underneath attached to the big metal brace that runs under the engine. Very easy to change. Realize the 2000 model year mounts are unique to your year. VOLVO S40 MOTOR MOUNTS 2000-2012 at Swedish Auto Parts

The next thing that comes to mind usually makes noise over slight bumps in the road. That would be broken sway bar bushings. These are hard to see unless you take a wheel off and have a good light. They are small aluminum/rubber pieces that hold the sway bar in place. When broken (and I've broken both sides) you will hear a metal to metal thumping noise over even the slightest bump. The dealer price was $25 each way back when.

The last thing causes a fairly loud knocking over bigger bumps. That would be worn out strut mounts. Take a look under the hood at the top of the struts. The metal threaded part that sticks out should be approximately centered in the hole. When broken/worn out the metal top of the strut is off center. Mine sounded like something behind the glove box was broken loose. It would knock hard over bumps. In this case you replace the strut mount and mount bearing. The 2000 is again unique to the year 2000. VOLVO S40 SUSPENSION FRONT 2000-2012 at Swedish Auto Parts

Other causes could be either drive axle although I've only experienced a "click, click" when turning a corner to indicate a bad CV joint. (aka CV shaft, half-shaft, constant velocity shaft)

Given your steering wheel shake I'd first look at the sway bar bushings.

 

Thanks, Hudini. I'll look at all those things. I just had tires put on today and they said my axle was leaking and needed replaced. Does that make sense with those symptoms?

 

It very well could but it's just a guess. By "axle leaking" I'm assuming the rubber bellows surrounding the CV joint is cracked and allowing the heavy grease inside to spill out as the axle spins. Over time this will surely kill the CV joint as dirt and water and salt get inside the joint and wear it out. My experience has been a clicking noise as you turn a corner but I've read others having to replace the one or both axles to stop a bad vibration at speed.

Try this also. With the car turned off, on a level surface, in park, parking brake off. Push and pull the car as far as it will let you. Watch the engine to see how far it rocks back and forth. There should be some movement but not excessive. You can also crawl under it and look at the 2 motor mounts for cracks in the rubber. I suggest this in case you have 2 issues where a motor mount is broken along with your CV joint wore out allowing you to feel excessive vibration.