Pipes watches Starscream do it. He doesn't want to try and intervene and dig himself deeper.

When he realises what Starscream is actually doing, though... he snorts then bursts into genuine laughter. The tidbits he'd been carrying back weren't anything really important.

"That was pretty good, I have to admit!"

He's not even consciously trying to appeal to Starscream's ego, he genuinely found it funny.