A couple of weeks ago I topped off my coolant to avoid the temperature spiking again. A little while after I noticed this noise like I'm driving through water. It only occurs below 3k rpm or if I let off the gas. At first it was only here and there, but now it seems to happen constantly. What's causing this and how do I fix it?

 

You have air in your cooling system. That what's causing the heating issue and the sound of running water which I assume you hear up under the dash. Bleed the system to release the air and you should be good.

 

Well I know one thing I'll be doing today

 

So I had the job done and left the garage and everything seemed to be in working order. Five mins later..noise is there again. Took it back to them and one of their guys said it may be a timing chain guide. Any thoughts?

Edit: Found this: http://www.6mt.net/forum/g35-coupe/3...-problems.html Describes my exact problem.