Metallic rattle when accelerating

My car has done it when it's been cold, especially on accelerating up the slip road on to the motorway after only been driving for a few minutes. Usually hear a noise from around 2750rpm.

10/15 minutes on the motorway and now onto a dual carriageway with traffic lights, I can't say I can hear any noise. I'd assume that's because the engine bay now has a bit of warmth in it.

It's only been doing it recently as when I've been coming out of work at midnight, it's been sub zero.

Can't say I've heard said noises in the warmer weather.