Is it more a sound caused by vibration? What does it sound like? We need more to help.

Let me clarify. Is the vibration low, high, clattering, squeaking, etc. What makes it occur? All engines produce some vibration even when everything is normal. You say it sounds like it’s under the dash. I would begin there. For example, I had a rattle that would occur only at certain rpm. It drove me nuts because the sound echoed as if it was coming from above the dash. I finally rode in the passenger seat and determined that it was the cheap Phillips speaker cone in the door that was rattling. Removed the speaker and no more rattle. It could very well be some component behind the dash that is loose.

First determine if it happens when the vehicle is stationary. If so then you can rule out drive train issues. Get an automotive stethoscope, fairly inexpensive, and start listening around under the hood and dash while someone works the throttle to try and duplicate it. Look at the damper on the crank shaft to see if there is any wobble. At only 1000 RPM you aren’t moving very fast so it’s probably something associated with the engine or something under the dash. Have someone drive the car while you sit in the pass seat and listen. These things are evasive but you can find it with enough effort. Get that stethoscope it will help tremendously.