I purchased a 1998 Mustang GT with 159K on the clock about 3 weeks ago. The interior and exterior needed some work, but I thought the powertrain was solid. I JUST transferred the title and registered so it figures I would start having issues now! Anyway, I am driving home from work and am accelerating in 2nd gear, I was not flooring it or being rough but I happened to let it rev to ~3500 rpm. At 3000rpm I heard what seemed to be a whirring in the engine compartment, almost like something was loose and shaking. It does not sound like metal on metal. It would start at 3000rpm, cut out between 3100-3400rpm, and come back at 3500rpm then I shifted. It wasnâ€™t loud or grating, but it was audible. The sound reminded me of a time I had a couple leaves stuck in a fan and they were rustling around. I was able to replicate this again in second gear later on in the night. It does not occur with 1st or 3rd. Additionally, I get some brief engine vibration between ~1300-1500 rpm. It is smooth when in gear and driving but if I am sitting in neutral and still and rev the engine the vibration will occur in that range on the way up and on the way down. I can feel the vibration in the steering wheel and â€œin the seat of my pants.â€ I wouldnâ€™t call it violent. The closest thing I can associate it with is the same feeling when my Acura had a lot of carbon build up in its intake and EGR valve, but that was only at idle, which was a very rough (and occasional stalled). The vibration with the mustang is not as severe. The exhaust does not sound consistent at idle, almost like it is coming out in spurts instead of a constant stream of pressure, you can even feel it if you put your hand up to the exhaust tips. I get the feeling that the issues are not related and it is just a coincidence that I noticed them on the same day, though I am not sure if that makes me feel any betterâ€¦ No check engine light is showing. Any help you guys could offer would be greatly appreciated, sorry for the length of the post and hopefully it is in the correct area. Thanks!