96 E300D Fan Area knocks When brake at Drive

[samu]

After a couple miles of drive and when come to a stop, either at stop light or parking lot, I can hear some kind of intermittent knocking from the cabin.

Had my wife drive. Back to parking lot, she holds the brake at DRIVE position. Open the hood, knocks is at the fan area. If the brake is released (while on D), knocks disappear. I used a HarborFreight stethoscope to listen to the area (alternator, pump, fan bearing, engine). No knocking. The only area I can't test is the rotating fan clutch.

Could it be the fan clutch need to be replaced or else? Strange is it only HAPPEN AFTER A COUPLE OF MILES OF DRIVE, BRAKE AT DRIVE POSITION.

Any suggestion?

[TMAllison]

On my 99, a collapsed left motor mount manifested as vibration at the serp belt tensioner. That vib can sound like a knock that gets better as the engine warms up.

You can release the tension on the serp belt (remove the spring) and fire up a COLD engine and run it for 30 secs to see if the noise is belt/accessory device related.

To check the motor mounts, use a rubber bushing between floor jack and oil pan and lift the engine ~1/4" and see if the noise/vib disappears. Dont lift more than that or you can damage the hard connections between engine and exhasut, fan/shroud and/or air filter housing.

Also check the bushings at both ends of the damper (shock); sometimes they fail which causes a clicking/knock.

[samu]

Originally Posted by TMAllison

On my 99, a collapsed left motor mount manifested as vibration at the serp belt tensioner. That vib can sound like a knock that gets better as the engine warms up.

You can release the tension on the serp belt (remove the spring) and fire up a COLD engine and run it for 30 secs to see if the noise is belt/accessory device related.

To check the motor mounts, use a rubber bushing between floor jack and oil pan and lift the engine ~1/4" and see if the noise/vib disappears. Dont lift more than that or you can damage the hard connections between engine and exhasut, fan/shroud and/or air filter housing.

Also check the bushings at both ends of the damper (shock); sometimes they fail which causes a clicking/knock.

Thanks Terry. I'll check it out.

[Matt L]

In addition to the above (which tells why it only happens with the brake), check your lower radiator core support. It is probably bent toward the engine, reducing the clearance to the fan blade.

[samu]

Originally Posted by Matt L

In addition to the above (which tells why it only happens with the brake), check your lower radiator core support. It is probably bent toward the engine, reducing the clearance to the fan blade.

Thanks Matt L. I'll check them out.
So . You guys do ntt thing the fan clutch has anything to do with the knock.

[spock505]

When the foot brake is applied with the car in drive it cause the engine to tilt slightly from the horizontal, this maybe enough for the fan to touch something -

You could take the fan off and try it for a bit.

[TMAllison]

Originally Posted by spock505

You could take the fan off and try it for a bit.

Much easier to release the tension on the belt so the fan doesnt turn; or just hold onto it so it can't spin.......

[spock505]

Yup, easier to loosen the belt I guess, those bolts holding the fan on can be a real pain to get undone -

I would be careful about holding it though just in case it is bust, if the fluid has leaked out of the coupling it could grab with metal on metal contact.