Thanks Phillip. I will check. The ones that were installed maybe also came from eBay I am not sure. Looking at the sensors, I don't see any abnormalities or paint defects. Everytime it rains, i get continuous beeps (loud!), which is very annoying. Weird, it doesn't happen previously when I washed the car, but only when it rains recently in Sydney.

So I plan to change the sensors but don't know which one or whether same thing will happen again after changing them.

Is it possible to reach the sensor from underneath the bumper by hand to remove it, or do i need to have the entire bumper removed? I don't mind to DIY if I need to, but to remove bumper is a bit of work there as I need space.