Unexplained Sound - puzzled mechanics !

[flex-appeal]

Hey all, this is my first post and im hoping that you have some suggestions.

Car is a Eunos 30X (MX3) 1995 Model V6, K8 Engine

Here is the situation, whilst driving along a perfectly smooth stretch of freeway a noise appeared from under the car. To me it sounded like perhaps a stick was caught underneath and was hitting the underneath of the car or something like that. The car didnt hit a bump or anything and was running fine so i wasnt too bothered. I came home and did a quick visual inspection but found nothing. As i was headed near a friends workshop i didnt bother looking myself i just took it in - he drove it and listened, and looked under the car briefly and couldnt see anything . He suggested that the problem may be diff/transaxle related.

The characteristics of the noise as as follows

-Sounds like a dull hitting, like someone hitting the palm of their hand under the car while you are driving . You can feel it distinctly through the floor and hear it too.
- It doesnt start until you reach about 50 or 60Kmph and increases with speed. Sometimes it stops altogether, other times it keeps making the hitting sound regularly as if it is in time with some rotating part.
-Engine is fine, everything works perfectly.
- Makes the noise in all gears, while accellerating, decellerating, neutral , and with clutch in or out. The only thing that affects it is speed of the vehicle.

The transmission mechanic drove it and said the gearbox is fine, and that we should look at the tie rod ends and CVs very closely.
The mechanic took it back and kept it all day, he had it on the hoist and removed the wheels and checked all the external components and says everything is fine.
I have had the car up on stands all day and taken off everything that could possibly move , and basically everything is quite solid under there. Tyres are good, alignment was done recently too.

The only thing everyone says is that there is definitly something wrong and dont drive it until you find out what it is!

Now i know its hard to explain sounds with words, but i hope that someone may have some suggestions , or info on common/potential problems with the vehicle.

[Grants]

Welcome flex-appeal!

Why couldn't you have an easy one to start with??? Damn Sandgropers.... Looks like you've done all the hard work here without any luck so maybe you're looking too hard? Are you sure it's not something blowing in the wind? Mud flap, piece of sound deadener, that sort of thing? Long shot I know but it's gotta be something....

[PATDIESEL]

Funny thing is that I agree. It sounds like something flapping around. I'd check all components for looseness. Secondly I'd check the wheel bearings. You'd think the mech would have checked them and you should feel that in the steering and the floor, but that could be it. To check the wheel bearings leave the tire on the car and then pull and push from the bottom and top of the tire, with the tire jacked off the ground and supported so that you don't push it off the jack. If it moves much the wheel bearings probably need to be replaced.

[timpronk]

Maybe a wheel wieght came off. A unbalanced tire could make a thumping noise only at certian speeds as well.

[jimishadow]

chec struts

[killerpickle]

What about the splash guards under your engine? Mine had a lose piece that was making some racket. Or a heat shield?

[flex-appeal]

So many replies already, wow you guys are great.. ok.. down to business :

I was also starting to think that maybe we are looking too hard, so i spent a long time looking for anything loose or flapping with no luck.

The plastics at the front arent amazingly tight, but they are about as flexible as i have seen on any car - so i doubt that can be making any noise.

In reply to your helpful advice

- Mechanic checked wheel bearings and they are ok apparently.

- I took front wheels off yesterday and had a look, they were fine, no clean bits where a weight could have recently dropped off.. I havent checked the back ones yet. will do that,

- Splash guards are all off so they are not the culprit.

Couple more questions

- id like to get a better look at the back, where is the jacking point to raise the whole back of the car with a trolley jack?

- how do i check the struts apart from bouncing the car? they seem ok, bumpy roads dont cause any extra noise etc. Although the rubber
strut boots are torn though

[flex-appeal]

Just to say thank you everyone for your helpful suggestions.

Finally we checked the car into the RAC service centre for fault diagnosis. They said Rt Wheel Bearing and/or CV.
This is very unnusual since the wheel bearing was the first thing that the mechanic inspected, it has no unnusual movement when the car is jacked up, and makes no noise when the wheel is turned.

In any case, today he replaced the wheel bearing and the noise has gone away, so it looks like that was the problem. Its nice that its fixed, but everyone is still confused how this thing that checked out ok turned out to be the problem. For future reference it cost $123.20.

Thanks again everyone!

[Grants]

Yea, funny symptoms but ah well - at least all is ok now! Thanks for posting back and letting us know.