Initially I thought this appears to be tire noise cause by uneven road
surface, because sometimes it goes away. But now it is becoming more obvious,
it sound like one of the tire hitting something on the road in every turn, in
a regular pace. One reason can cause this is a big nail on one of the tire,
causing a small portion of the tire to swell up. But I double check all the
four tires, there was no sign of any problem with the tires. This is a 91
multivan, front tires inflated to 35psi (Max psi), the two rear tires are
Capital 8 ply tires, I inflated them to 50psi (65psi max). Is there any
reason this could cause the noise? or could there be something else?

Any hints will be greatly appreciated

Albert
1991 multivan, 98,000mls