I can't help with the filter (but I can say they sometimes work
and often don't based on my past experience as a car stereo
installer). But, engine noise is uncommon on our cars so I would
look at the cause.

A common problem with 850s is a bad ground on the battery. This
can cause all sorts of weird problems beyond issues with the radio.
You should check the battery leads at both ends (useual cautions about
face protection when messing with a battery apply).

If it is a sedan, check that the antenna is well grounded. If the
noise is worse with AM that typically indicates a problem with the
antenna.

If there is a power amp make sure it is well grounded, too.

Pull the radio and then pull the antenna lead. The radio should still
work. If it does not, the radio is not properly grounded and is being
grounded via the antenna.

If you have after-market ignition wires then you might have to
replace them with OEM wires. A loose ignition wire could also
cause a problem. Check the spark plug wire caps at both ends
and the coil wire to make sure all are well seated.