2001 Mitsubishi Galant vibration after the timing belt was

Noises problem
2001 Mitsubishi Galant 4 cyl Wheel Drive Type unknown Automatic 112000 miles

I had the timing belt and the balance shaft belts changed last week along with the tensioners and idler. I also had all the drive belts replaced. The next morning, when I started the car, there was a very loud vibrating kind of a noise from the engine. When I drove the car, it was misfiriing, then we discovered there was an oil leak, so the valve cover gasket and the plugs were then replaced. The following morning, when I started the car, the noise was still there, and it has been there every since. The RPM is stable, doesn't move up and down, however at lower RPM the vibration seems greater. None of this existed before replacing the timing belts, could there be problem with the work done?