Knocking noise after replacing serpentine belt

[oldie]

As per the title. Belt replaced recently. Car left to idle this morning on start-up to clear frosted windows etc. Noticeable knocking sound from the front of the engine.

Picture of belt attached. I assume the item highlighted by the red arrow is a tensioner and is meant to flex to-and-fro. The knocking sound seemed to come lower down, maybe the blue arrow item? It didn't seem to be coming from the alternator.