I would check the suspension bushings, if one of them is worn it would cause a knocking noise.
The 1997 Ford Taurus has for motor mounts and to transmission mounts. Worn motor mounts and transmission mounts will allow the engine and transmission to have movement.
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A knocking noise can occur from worn engine mounts. When the rubber insert on the engine mounts are old and brittle the chassis is not protected from the weight of the engine. Creating loud vibrations that get louder as the vehicle goes faster.
3 total : Two engine mounts and one transmission mount.
3 2 engine mounts and 1 transmission mount
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Well I'll tell ya, I have a 1992 Taurus and the mounts just went out around 220,000 miles this November 2007.
Three two engine and a transmission mount .
I am still waiting for an answer. The book says on 1999 and newer Taurus's the engine and transaxle must be removed underneath the vehicle to change motor mounts. You will need a hoist and engine cradle. Big Job! Check with a proffessional.
There are a number of reasons why an engine would knock when you idle the vehicle. It could be your motor mounts. You may also need oil.
It is very stable due to the steel ball bearing mounts.