05 BXS C15 Ticking noise. Injectors?

I have an 05 Kenworth T600 with a BXS C15. I have it in the shop because of a slight Ticking noise that seems to be coming from the driver's side. You can only hear it while driving between 1000 and 1400 RPM. Above that it quiets down. It does not do it when idling even if you rev it up. The shop is telling me they think it has a miss and that I need a 6 pack of injectors. The truck does not feel like it has a miss and I've not noticed any smoke, although I have not had it under a load yet since I just recently purchased it. Any advice? Does this sound reasonable?

 

It certainly sounds like it, but I would imagine you'd hear that while sitting and revving it up to 1400 or so rpm. But there's no Ticking unless driving .

 

I was just driving it and with my head hanging out the window, it sounds like it's coming from the front of the engine. Maybe driver's side as well .

 

This is what i found...

Looks like soot build up around the exhaust manifold. Does it look like the gasket is bad?