I hope someone can help me with diagnosing a strange engine noise in my 84 300D.

Here goes - cruising at 60MHP on a flat road the engine sound is normal, or at least what I expect normal to be. When encountering the slightest incline, such as an overpass, and not varying the accelerartor any at all, it's as if a switch had been turned on and instantaneously I get a loud engine noise. As soon as the load is decreased on the downhill side of the grade, the noise instantly goes away. I' mean - its just like someone turning a switch on and off! At first I thought it was a loose exh. manifold but then it would be doing it all the time. My attention focused on the exh. pipe and all of its connections to no avail - all seemed tight. I don't see any readily evident exh leaks anywhere. I haven't looked at the turbo charger yet but suspect it may be in there somewhere since that seems to be the last possible place to look.

Anyone have any experience with such a phenomenon or suspect what it could be?

Thanks