As haasman said, it is almost certainly wheel balance. Loose suspension parts and the like don't MAKE vibration, but they will cause it to be more pronounced.
The biggest problems I've had with 123 cars over the years is getting the tire store kid to balance them correctly. They insist on using weights ONLY on the inboard wheel lip because people complain about wheels being scratched by the weights or they say the weights are ugly. It is always a fight for me to get them to do it right with weights both inboard AND outboard.
I finally gave up and bought my own wheel balancer.
If there are no weights on the outboard lip, find someone that will DYNAMICALLY balance the wheels with weights on both planes. Don't let them feed you any bull about inboard weights being just as good, that is B-!! $#!+.