The engine is producing a rattle, under load, only in the RPM range of 1500 - 2000. The noise is not a ticking, squealing or high pitched rub. It is as if ball bearings are rattling around or being shaken inside the motor. The noise mostly comes from behind the intake plenum.
The engine performs very well. Plenty of power, starts everytime and is getting 19-20 mpg around town. You can make the noise appear and disappear by going up a hill and feathering the gas to keep the van doing 2,000 RPM.
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Depends on your definition of rattle. Engines don't rattle unless something is loose or a rock has landed in a cooling hole on the side of the catalytic converter. So , if you can give us a better description of what the engine is doing, you'll get a load of incorrect answers.
Without a general description of what the noice sounds like, squeal as compared to knocking, it is impossible to help you. Engines make a lot of different noises, so try and help by describing noise. I would suspect it is belt related, or tension pulley related as these are RPM variable noises, belts tend to slip when you accelerate and can be quite loud with a serpentine belt as example.
no 4-3 or 3-2 downshift uncontrolled downshifts outside range of kickdown switch poor acceleration from stopped position parking pawl will not engage selector lever does not engage in r or p engine will not start in p or n position smoke in exhaust (fluid loss) fluid loss between torque converter & primary pump howling noise when changing gears (under full load) howling noise which increases as engine rpm increases 1st gear & reverse too loud 3rd gear too loud rattling noise at 1500 rpm in all positions except r light grinding noise in p & n positions rolling noises when driving in reverse vacuum control circuit Vacuum Control Valve (E300D & S350D) Vacuum Amplifier (E300D & S350D)
If your car has hydraulic tappets and low low oil pressure at engine speeds then it can make tapping noises. Oil pressure is needed to maintaine the tappets clearances, excessive cleaeances will produce noises.
Noise out of the engine. Noisy valves? Low oil level? Is the sound like tick, tick, tick, tick......that's how bad valves sound. I don't know if you have solid lifters those have to be adjusted after some mileage. Hydraulic ones are self adjusting. Only a good mechanic can tell for sure.
RPM and speed do not match what I'd expect to see at 120. but that could be gear ratio being different than I'd expect. Most don't like to run at 4k. Most problems at that rpm is either plugs (experiment with different gap settings or change heat range one step colder or hotter) or can be coil related (work fine except when recovery time is shortened due to rpm). However, the weird noise may be due to a partially plugged catalytic converter. So, check that as well. If you have a knock sensor, test that too.
Hello! First of all, congrats on owning a great car! That engine (3.8L Series II L67) is without a doubt one of the finest GM (Buick) has ever produced!
On to your problem - I think you have one of 2 things wrong:
1) a loose heat shield
2) a broken exhaust hanger
Both of these will cause intermittent (and highly annoying) rattling sounds, especially at low speeds or when the engine is under load (like going up a hill for instance). There are 2 hangers under the car located almost directly under the front floorboard. These are prone to breakage. There is also a heat shield in this same area - that is very prone to coming loose. If it's the heat shield - you can either remove it - or have it tack welded. If the hanger(s) are broken, you will need to take it to an exhaust shop (unless you know a welder :) ) and have those fixed - shouldn't cost you more than $20 either way.