Please Please help - noise from pipes under car

[Rattie]

Hi, I have a VW passat sport 1999 1.9 TDI. I have noticed a small knocking coming from under the car when it has reached engine temp or I have been driving for 5-10 mins from cold. If the engines warm the knocking starts only 1-2 mins into the journey. This noise has been getting progressively louder over the last few months. I think I have located where the noise is resonating from but its not the source - I don't think. Under the car on both the left and right hand side are 2 steel pipes side by side recessed into the floor (either side of the exhaust) now on the right hand side when the knocking is happening one of these pipe is vibrating at exactly the same time as the noise. The other side is not. Are these brake pipes - Could this be air in the system - I do not notice any braking problems. The noise may not be knocking it could be air but as its constant noise it just sounded pump like - the noise does not speed up with engine revs - its constant. Why would it only kick in after a time - it occurs when moving and stationary with handbrake on. Can anyone please help me - thanks in advance!