I have a 2009 cobolt when i put on the brakes slowly there’s a rattle noise and some shaking. The ball joints are new and so are the bearings. Any ideas could it be bad brakes or a strut?
My first guess warped rotors and a loose heat shield.
Can it be the strut?
Probably not.
It doesn’t sound like a strut problem, unless a strut mount is broken or the spring is broken.
I concur w/ @Barkydog above, most likely warped brake rotors. Easy enough for a shop to test, then you’ll know for sure.
My first guess would be a warped rotor or bent rim (hit any potholes recently?) making a loose heat shield or other loose part rattle.
My second guess if a bad rotor or wheel cannot be confirmed would be a possible bad wheel speed sensor causing pulsation of a caliper upon braking… since you’ve had recent work done, perhaps it could even have had the plug knocked off or left off.
Any special reason why the ball joints and bearings were replaced?
Have you had the shop that did the other work look at it? Did they render a guess?
How do i check the rotors?
ice69: it would be good if you answered TSM’s questions. It would help in developing a proper diagnostic.
Here they are again:
Any special reason why the ball joints and bearings were replaced?
Have you had the shop that did the other work look at it? Did they render a guess?
Checking the rotors for warping, if the warp is pretty bad, you might just be able to see it wobble side to side a bit by removing the wheel and turning the rotor by hand. There’s a gadget mechanics used called a “dial indicator”, that’s the usual way. It does the same thing basically, but is much more accurate. There’s a spec for the amount of side to side deviation as the wheel turns allowed. That would be a good time to measure the rotor thickness too, as rotors that get too thing from brake wear tend to warp.
If you are feeling like a scientist, another way to measure the side to side deviation of the rotor is to get a fine thread bolt, say one with a thread pitch of 1 mm, and corresponding nut. Figure out a way to make and securely hold a fixture, like with a clamp, where you thread the bolt through the nut on the fixture, so the tip of the screw just barely touches the rotor. Glue a paper dial to the other end of the screw marked in units of 1/10th of a rotation. Then you can adjust the screw so it just touches the rotor at one extreme vs the other, so you can then measure the rotor warp to within 0.1 mm. That should be accurate enough to compare to the spec.
It’s easier of course just to take the car to the shop.
A dial indicator is what a scientist would use… well, these days it would probably be a laser unit. Unless it was a financially-challenged scientist. 
A technical-type would also use a micrometer to check for variation in the disc thickness. I’ve seen variations so bad that I could easily feel them while spinning the disc with just my fingers.
I’ll spare you my bent rim stories. We have lots of potholes up here… and I have 45-series tires.