I have an odd noise on my 91 350 SDL and I need some advice on what it might be. When I am driving and I am making the engine work (RPM @ 3000) or 75-80 on the highway I hear a noise that I would think is a valve or rod tapping. But if I have the car parked and just rev it to 3000 rpm and hold it, the engine sounds fine. So this noise I am guessing has to do with either the transmission or something else just not sure what. I know describing a sound in writing is nearly impossible but if anyone can offer insight as to what may make an odd noise when moving I would appreciate it. Car has 138k on it......Thanks in advance for all assistance.......

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91 350 SDL - 155k and still purring
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