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The 'buzz' is the usual resonance that all 4 cylinder engines are prone to. It is inherent in the 4 banger design. Typically it comes in around 3000 rpm plus or minus. 2500 rpm on my car is nice and quiet and it is also smooth above 3500. In fact, it really hits its stride above 4000 rpm; it is louder, of course at high revs but nice and smooth with no buzz / booming. Around the 3000 rpm mark, there is a distinct resonance, a sort of booming. I have a wagon so that would emphasize the sound ... my own personal, portable boom box! I wouldn't be surprised if the interior dimensions of a wagon supported a standing wave in the range of 50 Hz (which is 3000 rpm), just the range for the low frequency buzz that I get.
The sound is not metallic nor the sound of anything loose. It sounds like the buzz that I heard on previous 4 bangers that I have owned, both Volvo and non-Volvo, wagons and sedans. The engine is running fine so I suspect that the normal 4 banger resonance force at mid rpms is being transmitted to the chassis and is amplified by the woofer like dimensions of the wagon interior. That's why I was asking about the motor mounts.
The car has 360,000 km on it so it may just be a question that all the various mounts are aging, perhaps the compression on the 4 cylinders are getting a bit uneven (I should check that out when the weather warms up)leading to increased unbalanced forces, etc.
Until I solve this, I will just drive at 110 kph or below to stay below the resonance. I'll save gas and maybe a life in the process. It is a puzzle though.
Bill
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Volvo Info Site 1990 745GLE 16valve
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