push fit or compression fittings for waste pipe

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Hi quick question
I used two 90 push fits bends to connect the 40mm waste to down pipe. Now when every hot water goes down it makes a knocking/creaking noise that is driving me mad. Guessing it is from heat expansion.
Do compression joins have the same problem? As I was just thinking of changing them over.
Also I check its not actually knocking on anything and it's not the soil pipe.
Thanks
 
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Sounds like the pipe or elbows maybe rubbing against something.
 
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Has 5cm clearent all round. I've even sat watching it, to see if it could have been hitting something with hot water going down but nothing moving and the noise still starts.
Also then put insulation around it but no improvement.
 
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Guessing it is from heat expansion.
Agreed. I doubt it's anything to do with the type of joint/bend. Somewhere along the pipe run there will be a pipe support/clip which grips/releases/grips/releases .... creating the sound.
 
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