Sound on pressing acceleration

[Mikastef]

Dear all,

My YARIS is a 1000cc engine which drives 100km every days, for six days a week. It has an automatic gearbox. I am approaching 100,000km and I am quite proud to say that this yaris is a very reliable car indeed...it has never failed me ....for now....and I plan to make it go over 200,000km...hopefully ...

I am very very disciplined in servicing it every 8,500km and do a lot of preventive maintenance. I change all fluids in due time.

My daily trip consists of 50% highway driving and the rest, a mixture of traffic jam and normal driving .

But lately, I am starting to hear some strange sound upon pressing the accelerator pedal. The sounds characteristics are as follows

-It occurs at random...say 40% of the time....not every time!!
-I noticed that it occurs mostly when I am driving around 30-70km/hr, and it is more pronounced when I am overtaking a vehicle and pressing more on the pedal. It nearly never occurs (or I don't hear it) when I'm driving over 90km/hr..I think when I put some serious load on the car in the 30-70km/hr range that I can hear it mostly...for eg. When I'm driving on a hill, overtaking, just slowed down after a hump and then accelerating fast to speed up..

The degree of occurrence decrease at night...since perhaps there is no load on the car as the road is very clear and I can drive at ease ( few stop and go)...it also never occured when I am going downhill.


-it seems to be a metallic sound (like pebbles shaking in a bottle lightly..or like a light metal rattling/hitting lightly against each other...)....-I am still trying to record it but it occurs randomly and thus is quite difficult to record...
Once I get a good recording of it, I will post it.

-I feel that the sound occurs in the engine bay, in the driver side corner (where the water pump, accessory belt etc are found ). Sometimes my ears and mind plays me tricks and I feel it is at the back of the car


I have made some research online and some pple say that it may be due to
1. Heat shield which is loose -nope.i made a mechanic checked my heat shield and apparently it is good and not loose.
2. It relates to gas octane ratio ..- well I have changed fuel stations so many times. And it random occurrence makes it difficult to know for sure if it related to this problem.
3. Transmission problem - I pray that it is not.....!!!!!! But I don't get any gear shift problem. My transmission is smooth.
4. Loose Vacuum or dirty throttle - well I didn't check the loose vacuum yet.and some 30,000 km back I did make the mechanic cleaned the throttle.
5. Brake pad - i still have some brake pad left.
6. Axle or cv joints - nope. I don't have any problem when turning. My car is smooth when I'm turning.
7. Water pump -but it occurs mostly on acceleration...so how is it related to water pump??? And my car temperature seems ok..ranging around 85-95 degree. I plan to change the water pump when I reach 100,000km.
8. Bearing - wheel? Other bearing near the water pump?? I don't know..didn't check yet.
9. No error code either through OBD Scanner
10. Timing chain going bad- my god...this is going to cost a fortune to change ! I hope not..the car is still young...

Please help me...
I noticed that mechanic sometimes does not know your problem exactly...and it is up to us, the drivers that give the mechanic all the clues for a successful diagnostic. Else we run the risk of spending lots of time at the mechanic or, the car may get damaged eventually due to bad diagnostic .