Noisy old Jetta

Maintenance/Repairs
1

I have an 88 VW Jetta that makes a pretty loud roar from under the hood anywhere above 25mph, becoming louder and more high-pitched with speed. At first, I thought it might be a leak in the exhaust manifold or something. But that can’t be if it doesn’t matter what gear it’s in or how high the engine is reving right? Even if I coast at 1rpm, the noise is the same. Help me out!