I am having an issue with my drain pipes. When I run hot water, either through my kitchen sink, bathroom sink, dishwasher, etc., there is a knocking sound that comes from the drain pipe. [B]IT IS NOT THE SUPPLY LINE MAKING THE NOISE[/B]. Just want to be clear about that part. It will start off as a rapid knocking sound that will gradually slow down to a stop in about a 2 minute time frame. But after a while of constant hot water running, it will go away. However, the moment I switch to cold water, the knocking sound will come back in the drain line. It sounds like it is coming from behind the wall. I do not have ABS pipe for my drain. I believe they are steel. Is there anyone else that has had this problem and knows how to fix it?

So for the past few years ive steadily noticed the moulding beneath my bath begin to rot and warp. The half bathroom on the other side rarely gets used, specifically the sink. That is why its taken this long to actually do something about it. I thought somebody may have knicked a pex line when mounting their decorations on the wall or maybe some abs fitting came apart. Yesterday night i began with tracing the source of the leak. I removed all the rotted moulding and started with the hole on the left. I was thinking the pex lines run to the sink on the other side or maybe the abs drain line for the sink was leaking behind the wall. Nothing was found to leak so i followed with the hole to the right. There i spotted the water pooling right underneath the cleanout on the profex clamp. I was snug but i still managed to tighten it another turn to turn and a half. I was hoping that-that was going to be it so i left it overnight to dry. This morning i came back to it to test it. I turned the faucet on and it slowly began to leak again. I noticed there was some ridges of what feels like cement on the back of the cast iron piping where the proflex connects to. I grabbed some 80 grit sandpaper and tried to sand it down and reconnected the clamp super tight. The cement would not even chip off so it must be some good stuff stuck on there. I tightened the living daylights out of the clamp and it still leaked. My last resort is opening the wall on the other side to try to chip off the cement better if thats whats causing it to leak. If i open the wall on the other side what are the chances that the pipe is cracked and what can i expect to pay to repair it. IIRC this leak has been there since the house was turned over to me 3 years ago after a total ground up fire restoration. [ATTACH]51474[/ATTACH]