Did you use wheel spacers..? how about the wheel balancing & alignment..? I think the problam is with tier wide or alloy wheel size..? what is the size..? and tier size..?
what amount of air are you using for tiers..?
Sometimes If we used different tiers/alloys, it can't act like a original machan.. I know some people are suffering by problems like this because of arrangement of their suspension system or the alloys, but most of those sounds are just a noise. it is no any harm for the van or tiers.. so first you should check wheel balancing/alignment. then suspension errors, excel racers... and tier air pressure..? If all ok.. it is just a noise, coming from those wide tiers..!!
but specially, be in touch your excel racers If you are drive with big loads on bad condition roads....! If you can, avoid them, If it is can not... drive slowly...!!!
I mean big load with bad roads & high speed is a not good combination.... !