I've been browsing the forums for a long time now searching for answers to other little problems and I ended up finding them most of the time. This time, however, I cannot find anything that fits my problem... It started happening maybe one year ago (yes, I've been looking for a solution for that long and not even at an opel dealership in portugal were they able to help...) As soon as the car goes to 5th gear and reaches about 100 km/h (before my latest big trip of 3000 Km it used to happen only at about 110-120, now down to 100) there is this strange intermittent vibration sound coming from the front of the car. It's as if the engine is trying to accelerate or decelerate but I don't feel it power wise, plus the fuel consumption seems OK (about 7L/100 km in a 3000 km trip of mostly highways between 110-130 km/h).
I've now put it running on LPG (car is 193.000 Km now, 190.000 before conversion) and it still does the same. To remove the usual suspects (tires, suspension, etc) it suffices to say that when I put it in neutral or simply press the clutch at those speeds it becomes silent. And I know about the easy fixes: turn on the radio, open the windows but i was looking for a more definitive solution. I don't really know how to put it in words, tried recording the sound but it is not perceptible. It as if the engine does vvvvvvvruuuuuumvvvvvvvvvvruuuuuum in maybe 1 sec repeats that get slight faster as speed increases (being the vvv the normal sound and the ruuum this weird sound on top of the normal engine noise). Beyond 140-150 it disappears because the engine becomes too noisy.
What could it be? At Opel they said "oh, it's lambda sensor number 2" so I changed it, wasted 150 euros and nothing! The exhaust is still the original but I went to get it checked and the guy said it was fine (he could easily have said i needed a new one but he was pretty honest about it).
The engine has always worked fine, smooth, and only burned a set of coils, had the sparks replaced once, plus the normal service of belts lubs etc. I hope this is not something that is ruining it...
Any help, PLEASE?????
Thanks you in advance!!!!