There are times you have to knock on the ceiling several times. In Manila, it is the accepted way, but there are drivers who are against it. You have to knock on the ceiling particularly if the jeep's stereo is blaring. Another occasion is when the jeep is long and you are at the opposite end of the driver or near the entrance. You word "paraaaaaaa" could not be heard by the driver. Of course it os impolite to knock hard for other passengers could be disturbed. But if it's the only way you could be understood, you may knock on the ceiling-- but once.