Knock when I hit the gas or break

First thing, change mechanics....sounds like you have a bad motor mount or transmission mount. Every time you punch the gas, your engine and tranny want to twist. A bad or cracked mount will cause a noise like that to occur. Have someone with some mechanical experience slide under your car and take a look before letting a shop look at it. I never trust any of those guys. If possible, put the car on 4 jack stands. Put the car in gear and punch the gas. See if you and a couple of buddies can replicate the sound. Have one guy look under the car while you hit the brakes and hit the gas. If you can't replicate the problem, I would start looking at the suspension. You may have a collapsed strut, or a broken coil spring. Post back with findings......