My first 240 developed a serious knock at something like 300K miles. Got worse fairly quickly. Would knock louder, related to engine load.

Mechanic checked everything - finally did a compression test. Bad in cylinders 1 + 2. Head gasket was blown out where those two cylinders come close to each other. Combustion pressure leaked into the neighboring cylinders pressing on piston in the wrong direction, thus the knock.

Fortunately a compression test if fairly easy to do. The tester is low cost if you don't have one. I'd test that before examining everything else.

Good luck!
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Sven: '89 245 NA, 951 ECU, expanded air dam to 7" from ground, forward belly pan reaches oem belly pan, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245 NA stock.