Pedal bearings shot, or loose crank/pedal.

See here.

Fix? Replace it/them, check crank and bottom bracket play while you got the spanners out.

Tern are having occasional problems with Stomp pedals. They're made by usually acclaimed 'Wellgo'. There's an entire thread devoted to folding pedals here.

The usual answer is either to buy no-name $5 replacements from Ebay every six months, or splash out on expensive alloys which may or may not last longer.