360 vibration at idle standstill

Strange thing today. Crawling in appalling London traffic, I start to get a significant vibration (like the buzz of a pump) from time to time. Really noticeable through the brake pedal, although audible and (just) tangible with feet off the pedals. Car in neutral, foot on brake. The vibration lasts a few seconds, then clears. To give you an idea of what it feels like, I thought initially the engine was idling faster, but the rev counter was steady, and concentrating on it, it feels like it's coming from the front of the car. All gauges are nominal.

I suppose my instincts would be ABS pump or a fan problem. Any thoughts?

Fortunately the car's going in to Verdi tomorrow for a check-up, but it would help to be able to steer them in the right direction (and to not worry on the way home).

2003 360 F1 spider, by the way. Bone stock.

 

Yes. You think it could be the compressor? Edited to add: I've not felt it before, it's pronounced enough that something is definitely wrong, and it's only really noticeable through the brake pedal. Where is the compressor mounted? Or is there a separate fan for the a/c radiator? And where is that mounted?