2001 Subaru Outback chatter

Just dropped my Subaru off at the trusty local shop. 4 cylinder, 5 speed manual and when driving under load there is a quiet chatter, maybe clicking noise which disappears as soon as I release the throttle.No chatter when coasting in gear or neutral but also chatters when going downhill and engine engaged. Valve lash adjuster, or some animal stuck under the hood?!

Forgot to mention 126,000 miles on it with great maintenance. Thanks.

My local garage charged me $30 to cut off the shield several years ago. I resisted because the noise embarrassed my wife so badly, but finally had it taken off. There have been no problems since removing them.