1998 GM LS 131k miles

I sometimes hear rushing water from behind the passenger side dash when making turns or going over speed bumps. AC/heat work great. Is this air in my heater core? I cant seem to bleed any air out by opening the coolant reservoir tank and letting the car heat up. Is this okay to let be? Ive had this sound for a while without any issues. Thanks

 

Check the cabin air filter? Mine is located on passenger side and they can potentially let water in.

 

The water is under your carpet. You need to let it out using a plug in the backseat passenger footwell. You'll have to pull up the carpet. First you'll need to remove the back seat, use your knee to push against the seat while simultaneously pulling upwards (this should not take very much force). Then pull the carpet up and find the plug, it should be in the corner. The water in my Marquis came from the cowl drain. The seal had deteriorated allowing water to flow backwards behind the firewall and under the carpet.