problem of engine sounds
 

hi,
i have a 2008 mazda 3 sedan GS with sport package and auto transmission.
i have a question to all of you.
please answer my question if you know the cause of the problem.
i usually drive it within the city with trafiics all the time.

here is the problem i have on my car.
when i drove on a slight hill and my transmission is on 4th gear if i don't step on the gas pedal further, the RPM drops for engine to take less fuel.
then, i can hear a sound like water or liquid flowing in the engine or somewhere within my car. (i would say the sound is from engine area)
it also sounds like light diesel engine sounds.
i hear that sound when i start to drive out in the morning (when the engine is cold)
i can even keep hearing the sounds no matter the engine is cold or hot.

thanks for any help...

The water sound is classic for low coolant, where it will be pumping water through an air cavity at the top of the heat exchanger in the cabin... think it's dead center in the dash.

The other option would leave water on the floor, which is a plugged drain for the AC system, so water will accumulate in the cabin and would slosh around when driving.

As for the diesel sound, you mean clicking / rattling like a diesel?
The fuel injectors can make a lot more noise than you think - especially if you replace a fuel pump or something the higher pressure would make it louder. Though the engine valves tick a little as well, at least I think that's what it is.
You can use a stethoscope, or a piece of tubing held to your ear to put on items and track the sound down that way.

Good luck

i test drove with a technician from mazda delear and after that he asked me to use higher octane rating gas and top up the coolant.
but still nothing has been changed after trying those methods.

it's not diesel noise then, morel likely water flowing sounds. (the sound is like water flowing inside tubing or something)
hmm...
i still don't know if this is even normal thing for mazda 3 or not.

Was the coolant low? I know it kinda stinks that we have no real radiator cap to check, but any of the tubes you should be able to listen for water / air mixing when the engine is running. This sort of sound (water) is not normal - don't let the dealer convince you that it is.

Make sure that no water is getting in the cabin. Also you can pour water on the windshield and make sure you see it drain out below - that drain could be plugged allowing water to slosh around as well... easy to get plugged with pine needles or small leaves.