So I replaced an idler pulley (smooth one). Found a Dayco replacement and transferred some of the parts from old to new (washer, bolt, bearing spacer)

Started to make noises when cold and under pressure. Slowly got quieter as the car warmed.

Lived with it for a few weeks and was not getting better

Bit the bullet and got an oem replacement. Problem solved

Lesson learned - use oem parts on important parts of the engine. Worth the extra cost.