no idea why, but ive got really tight right calf muscle this morning.

should I go for a gentle run at lunch to ease it, or better to rest it?

yes, but stop if it hurts 🙂

make sure you stretch it afterwards too though!

I wouldn’t, but please go ahead and then post here afterwards:
http://singletrackworld.com/forum/topic/dumb-self-inflicted-sports-injuries

Depends on how tight “really tight” actually is. But I’d probably go for a walk rather than a run if it’s tight enough to hurt.

You shouldn’t run. Ever. (Ice cream vans may provide a single suitable exception)

I’d go for a ride on a tight calf muscle, but not a run. Eccentric contraction in running which can result in a bad tear if your muscles tighten more (been there, done that, though with muscles which got tight whilst on a long run rather than starting with one).