谢谢楼上的，这2天做了些research，一个权威的plumber也是这样解释的，属于正常现
象。
What is happening is very normal.
Plastic expands and rubs against wood. This sort of thing has been going on
for decades without an issue.
A Furnace will have simalar issues. The pipes warm up, and then they cool.
As they cool things slip back into postion again.
A home that faces the South in the Summer, will get taller on the sunny side
. In the evening, after the sun goes down, the house contracts on that side,
causing all sorts of sounds as the different pieces rearrange themselves.
I notice that in the Winter, after the leaves have fallen, I hear the
freeway better at night. I don't notice so much in the Summer with more
leaves between me and the freeway a half mile away.
