loud groan / whine noise at high rpms (over 4k) after letting off the gas. Remains shortly after shifting gears

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. I'm sorry to hear you're having trouble with your Mustang. Based on the symptoms you've described, the noise could be related to several things. Since it goes away when the clutch is engaged and comes back after shifting, the issue may lie within the clutch or the transmission itself. A worn-out or failing clutch release bearing (also known as the throw-out bearing) is a common culprit in this situation. The bearing is responsible for disengaging the clutch when the pedal is pressed, and if it's worn, it can produce a groaning or whining noise, especially under load or high RPMs. Another possibility is a problem with the transmission components like the input shaft bearing or even a bearing within the gearset. Given the slight rattle/grind sound, it's also worth checking the gear synchronizers, which help the gears mesh smoothly. If the noise persists after shifting, it could indicate internal issues with the transmission. Pulleys, on the other hand, tend to cause noise while the engine is under load (such as accelerating) and are typically more constant rather than tied to shifting or clutch engagement. However, it's still worth inspecting the belt system, including the tensioner and idler pulleys, just in case. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!