Harmonic Resonance at 2500rpm

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Joined
Sep 9, 2017
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18
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Location
Plano, TX
So I recently acquired a great ‘99 LC with 135k on the clock and have spent the last month or so doing the usual deferred maintenance and preventative maintenance and with each project the truck becomes more and more “mine”. I recently noticed (after a DynaMat project) that there is a harmonic resonance from the engine at about 2400rpm. The resonance goes away when the engine gets to about 2500rpm. It is not noticeable driving around town but is noticeable at freeway speeds. 2500rpm coincides with 70mph in the freeway which is a common speed around DFW. The air filter is new and MAS has recently been cleaned. I’m doing the timing belt at 140k along with the normal “while you at its” including plugs. Has anyone noticed this or is this a symptom of a problem? It’s not a vibration: when I rev the engine to those rpms the motor is butter smooth which rules out the driveline.

Any suggestions?
 

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