'Pulsing' Mondeo Engine - possible transmission issue?

[456GT]

Here's the scenario:

I'm in traffic, first gear, handbrake up, clutch down. The engine starts pulsing very slightly. The revs blip themselves to about 1k and drop back to idle, around 700-800. It keeps doing this at a rate of about one per second. If I drop it into neutral and take my foot off the clutch, the engine returns to normal, no more pulsing.

Is that a clutch issue? If so, how serious...?