This can get very tricky as to perceived ride quality and noises. We rebuilt a machine twice for a customer that did the same thing and it turned out to be a cover on the florescent light that was vibrating that we fixed with some weather stripping!!! No Kidding he heard a very faint buzzing from the cover and equated it to poor rider quality. There are testing devices that can help and you can equate the results into data that can tell you if its roller guides or bearings or machine issues but someone has to explain to your customer that you are moving tons of equipment and that a totally vibration free ride is unobtainable. But we have spent weeks trying to get the problem car to run as smooth as the best car in a group!! What we do is to preform vibration and noise readings in the cars before the mod starts. This will help later when you turn the car over and someone THINKS its now noisier than it use to be. A real problem on hydro mods especially when you add rupture valves. Good luck as I have been in your shoes. My best advice is to start with your cab steadiers and make sure you have the best isolation you can come up with and that the car top hand rails or any other devices or duct doesn't tie to cab into the car sling.