Subaru rough idle

#1

I have a 2003 Subaru Outback, 78,000 miles. It idles very rough while in drive, not in neutral or park. My mechanic isn’t sure what is causing it. Any ideas?

#2

The first step is to always rule out a mechanical fault and that means a compression test.
If the compression is good then there is not enough info provided to make a guess.
Plugs, wires, coils, vacuum leak, faulty injector, bad mount, Check Engine Light on, codes pulled?, etc, etc, etc.