2006 Ford Focus Wagon suspension

Hi, you could check your engine mounts.

I had an issue with the rear sway bar links that caused a knock and rattle, the sway bar keeps tension on so it is hard to feel for play.

Remove the front wheel and check the wheel bearing, sometimes you may have to may have to put a bar under the wheel and try to lift it up hard, this will give you an indication on where any movement is. THERE IS A PROBLEM and you need to find it

I just bought a preowned CLS 500 and recently noticed the same issue. Whistling after I start and stop the engine. "Car too low" notice if I hit a speed bump too fast. Etc.
I'm guessing it has something to do with a leak in the air suspension.

Did you ever resolve your problem?

It's the anti-sway bar endlinks. My '06 g35x has the same symptoms - clunking at low speeds, varying from which side of the car the noise comes from as to which side of the car is hitting bumps in the road surface. EASY to replace yourself with the front end on jack stand, and you don't even need to remove the underbody plastic panel, so it's quicker than an oil change. To verify this is the problem before you order the parts (which are about $100 for the x, as it's a different part altogether than the endlinks on the RWD sedans or coupes, so be careful when ordering), just remove the current endlinks and take a test drive - don't worry about removing the whole sway bar, just the links at the ends of the bar). If this is the case, you will not hear the clunking any more. I do not recommend driving like this for any distance, as the car will become tail happy and your sway bar ends could point towards the ground while left unsupported and become damaged.

lots of possibilities here. have you recently hit anything such as pot holes? try rotating tires from rear to front and see if there is a difference. Beyond that it could be anything from bent wheels to out of balance tires(less likely to be noticed at low speeds), or perhaps bad tires(choppy tread). Next is to check steering and suspension for worn or damaged parts.
Hope this helps.

catalytic converter is probably rattling, they do that when they are on the way out, performance will be effected, as it block's, engine cant exhaut properly !

its your muffler hitting the bottom of your car, and also the way that our exhaust is set up is it goes above the suspension in the back so thatt might be hitting too

first try to tie all your bolts in the front end with1/2 drive wrench,then try to bounce your car in the front to pinpoint where is the rattle it could be your strut mount