how do i stop the shuddering and hissing noise from my renault megane coupe?

hi there!
well i have a renault megane coupe w reg (2000)
i just recently bought the car, i've also recently had a major service and diagnostic done on it with everything coming back as okay!!! but i've noticed the car seems to shudder while idle (with the rev counter moving up and down and gear stick moving!), and also it seems like there is a hissing or some sort of noise from where the tensioners are, and looking carefully the engine seems to move!!!!!!
so i took it to a few garages and the looked it over with no success and different storys, but one garage noticed the stabiliser holding the engine was worn so they replaced it with a new one but it still seems to shudder and make a noise but not as bad!!!!!! and one final thing there doesnt seem to be a smooth sound from the engine like other cars it sounds as if i'm revving the car!!!!!!