Engine Rattle at Constant Acceleration

Question

2005 Subaru Legacy GT 5MT

Having an issue where my Subaru feels like I'm driving over a constant rumble strip. Thought it may have been a spark plug issue but changed those out and still not fixed.

Here are the symptoms:

1. Most noticeable during acceleration/deceleration (applying/letting off the gas) and primarily on non-hilly roads, at higher speeds
2. Is the absolute worst when on cruise control coasting on a relatively flat surface at 50-70 mph
3. Not noticeable in neutral while coasting or while going downhill

Answer 1

Turned out it was the driveshaft.