I replaced a leaking waterpump as well as the drive belt and balance shaft belt. I was careful to get everything lined up to top dead center when I put everything back together. Now the engine vibrates or buzzes at low rpm's. All nuts and bolts were accounted for. Now what?

 

make sure all bolts are torqued down to spec, if they're lose it will vibrate

 

are you certain the balancer belt is on correctly. if the crank wasn't at TDC when you set the balancer shafts you could have the belt on right, but the balbancers are out of sync with the motor