"thump" under dash when wipers going

When my wipers are going (not intermittently) I noticed a “thump” that seems to be coming from the dash. It’s not the blades hitting. Sounds like someone next to me has their stereo too loud with a fast bass underlying the rhythm of the wipers. Of course the garage guys couldn’t hear the sound when I left it with them. I can’t hear it while standing outside of the truck. Any ideas?

Perhaps the wiper motor linkage in the engine compartment is hitting something.

If the noise is there constantly why didn’t you get one of the mechanics into the vehicle and have them listen to what you hear?

The noise isn’t constant and it’s not loud. There seems to be a delay of a few seconds after I turn on the wipers before it starts. I’ve pretty much decided that the next time it starts I’m going to pull over and pop the hood and see if I can hear it better. (Hopefully when it starts raining again I won’t be dressed for work and it won’t be pouring too much!)

If you can’t find anything hitting, maybe it’s a loose joint in the linkage.