Another newbie here but have utilized much info from the discussions I've read. I have a '08 Mazda 3 with 97k miles. I have a strong vibration from 45-65 mph and only when accelerating. When coasting or out of gear the vibration disappears. I feel the shake most often when accelerating onto a highway or when increasing speed to pass another vehicle. However, when I reach a speed and maintain or cruise at a speed of say, 55 mph, then the vibration is reduced. The tires are balanced and I can only say that I have checked the cv boot for visible issues which I can find none. I have driven in a parking lot and have turned sharply to each side and can hear two knocks on each turn but nothing severe, both in forward and reverse. Anyone out there experience this? Im betting so! Any info would be greatly appreciated. On another note, I have also noticed a leak in the left rear of the trunk for whatever reason, showing up when we get a good Oklahoma rain!

 

I had the motor mounts on my 2008 Mazda 3 when I had a similar situation. Fortunately, mine was still under warranty at the time.

 

Sounds like it could be the CV joints. The hallmark traits of that are a knocking sound that occurs when turning and accelerating. It may happen when driving straight, but is less common. It's really not going to happen when coasting or braking, or even generally under light acceleration.

If it is the CVJ's and they're far gone enough to be knocking then you'll need to replace the front axles. You said you checked the CV boot.... did you check both the inner and outer ones? Did you just do a visual inspection from next to the car or did you get underneath it? A torn boot would probably show signs of having sprayed it's grease out around itself, so look for evidence of messiness. Also if you can put hands on all the boots you can feel around the creases and see if there are any tears.

As far as the motor mounts it's possible one or more are gone and the knock you're feeling is the engine actually shifting and clunking into things. The most common mount failure is the passenger side mount. It's clearly visible between the engine and the coolant overflow tank. Again look for any signs the fluid inside has leaked out (would look black and messy on/around/below the mount). You can also poke your finger at the mount and see if it's still soft/pliable. Be gentle since you could stick your finger straight through it. In the case of mine I barely touched it and my finger went straight through it because it had already broken, leaked the oil out, and the rubber had dried and become brittle.

 

If it is the MM then the Mazda part is $80-100, and it can be replaced by you. You just need a jack (preferably a floor jack), sockets/breaker bar. You need either deep sockets or an open wrench for two of them as the studs are long. Just support the engine with the jack, remove all the bots, swap in the new mount, and tighten everything up.