1997 Legacy 2.5 GT Strange Noise and Check Engine Light

I have done a lot of maintenance on this car over the past year to bring it up to snuff.
A year ago - complete engine reseal
A month ago - brakes and new alternator and battery.

A couple of weeks ago encountered a CHECK engine light with codes P301 and P1507. Engine idled roughly and stalled once while driving.
#1 cylinder coil and idler control valve replaced.

Ran fine for a couple of days then encouterd CHECK light again and rough idling. This time dealer found leaking intake manifold gasket.

Again ran fine for a couple of days. My son had the car out last night and again ecountered rough idling and also a strange noise while driving that disappeared when off the gas and slowing. CHECK light again on. I haven't heard the noise but he describes it as sounding like a "Go-cart". The physical symptoms disappeared while on the way home but the CHECK light persists. The car seems to run fine for me.

Any help would be greatly appreciated. I'm considering taking it to a trusted independent mechanic this time for evaluation. I'm also wondering considering the uniqueness of the engine design if this would be a good idea?

Thanks
Lynn