Steve, That's NOT the way a shutter's "bulb" mode works! The nomenclature goes back to the days of pneumatic remote releases that used a small piston to actuate a plunger on the shutter. The air pressure was provided when a rubber "bulb" was pressed - a length of small bore, flexible, tubing connected the bulb to the piston. The pneumatic release could/can be exchanged for a MECHANICAL cable release for remote distances of up to about 4ft.
With shutters on film-based cameras there were (still are) "B" and "T" settings. The B setting causes the shutter to remain open while the release is pressed, closing it again when the pressure is removed. The T setting works like an on-off toggle: A press-and-release opens the shutter and LEAVES it open until a subsequent press-and-release.
By your description, you are expecting the shutter on your D7000 to act as if it's in "T" mode - but the camera only offers "B" mode. In fact, I don't know of any digital camera which offers a "T" mode shutter release.
The effect you describe is completely separate from any noise reduction setting - and is EXACTLY what the camera is supposed to do.