Here’s a crazy one. This ONLY happens in heavy, flooding rains where there’s a couple inches of water or more on the street. My 2003 Pontiac Vibe GT (manual 6 speed) creates a rumbling noise like someone is under the car banging on it – it sounds like thunder. It seems like the car loses power – but I have to say I can’t help but let up on the gas because as the car slows the rumbling stops. What it sounded like, and what i thought it might be, was that deep water was catching a loose plastic under panel and flapping it. My mechanic could not find any lose panels. Someone told me the Dodge Caravans had this problem in rain as the timing belt slipped. The Vibe has a timing chain. It’s a hard one to recreate for a mechanic and when it happen you are already white-knuckled from the flooding rain. Any thoughts? Thx.
The reference wouldn’t have been to a Dodge Caravan timing belt slipping as they also use chains. Timing belts also have teeth. It would have been to the accessory belt slipping - which would be my first guess. Those “plastic panels” are splash shields that are there partly to keep junk - like water and other debris - from getting up into the belt system. My first guess would be that water is getting up between/around the splash shield and soaking the belt and crank pulley (at the very least). The crank pulley is what drives the belt and thus all of the other accessories. I’m trying to figure out why this would make a rumbly/thunder sound though. Of course, it could just be what it does sound like when water pounds on those shields and thus it is just those making vibrational noise. They may not be obviously loose, but most of them are quite flexible and have a lot of room for movement.
I would crawl under and re-check the panels. At speed the panels could be pulling down in the wind and scooping up water. Throwing it on the belts and or shorting out the alternator.
Check the plastic wheel well liners.
My guess would be simple water splash on any of the plastic panels. most likely the wheel well liners. If it’s really “a couple of inches of water” you re driving thru, that alone is enough to slow a car. And any seeming lose of power may well be your front wheels hydroplaning. Once the float, they aren’t putting any power to the road.
The well-documented problem with Chrysler minivans is with the serpentine accessory belt. They are even known to pop off the crank pulley (ask me how I know this). Make sure your serpentine belt and tensioner are in good shape, and all splash shields are fully battened down. One loose or missing fastener could cause a problem.