Knock.... Please describe further!
Please tell us more about the knock - does it sound like a "tapping" noise, or is it a heavier sound, like a "klunking" noise? Can you feel it through the seat when it's occuring? Does it sound like it's coming from the engine, or do you think it's coming from the rear of the car, or maybe the transmission or driveshaft area? How warm does the engine get when the knock disappears? Does it only occur under heavy acceleration, or does it also manifest itself under light throttle? It could possibly be coming from a number of places, from a rattling tailpipe to a rod knock or piston slap, or a bad center bearing, or even a bad motor mount, so even though the questions might not seem important to you, they might be very helpful to the people reading your thread so they can zero in on an accurate diagnosis. SO, what I'm trying to say is this: even though it might seem insignificant or even dumb, please try to answer the questions, and lets see if we can be of some real help!
Mechanic for over 45 yrs
1982 Mercedes 300D/4.3lV6
1977 Jag XJ6L
1974 Mercedes 280C