sometimes out of nowhere my car makes a loud whirring noise under the hood...I pull over as soon as I can..and it slows as I slow and stops when I stop...sometimes it stops before I completely stop. I replaced the tension and idler pullys & motor mounts. .it isn't brakes..
My car has an automatic transmission.
What you are describing sounds like a wheel bearing that may be starting to fail. A wheel bearing will usually fail due to pitting or small damage on the surface of the rollers or the bearing race. Both the surface of the rollers and the race is precision machined to tight tolerances and highly polished to allow the rollers to pass easily over the race with the addition of bearing grease for lubrication and cooling. Over time the bearing will wear slightly, allowing microscopic pieces of metal into the grease. These microscopic pieces of metal, as well as any contamination that gets into the bearing, will wear away at the highly polished surfaces causing pitting and deformation. Since so much weight is riding on your wheel bearings, and so much load is put on them in corners, even the smallest amount of surface damage to your bearing can cause a lot of noise. When a wheel bearing is damaged or worn out, it may produce a bit of a grinding or howling noise that will be more noticeable with speed. I would suggest having an expert from YourMechanic come to your location to diagnose and inspect your vehicle.