The " slicker " is gonna come as algae coats the inside a bit ... the difference in sound between PVC and ABS would be in harmonic resonance ( thickness / density etc ) ... just wait awhile as Jonathon said and it will quiet down or use flexible tubing like I do and reduce the number of 90's that way
Cheers
__________________
Steve
“The most important decision you make is to be in a good mood.”
― Voltaire