If I'm understanding you correctly, using a different length 'whip' for an antenna without also changing what that 'whip' connects to (the base of the antenna) just isn't going to work. That base of the antenna usually contains a matching device and/or load, which uses whatever the original length 'whip' as part of a tuned circuit. Changing the 'whip's length from 38" to something like 15" will mess up the tuning, not gonna work well or at all. Go back to the original length and see what happens. (I misunderstand a lot sometimes, so if this isn't what you meant, I didn't really say any of it, forget it.)
Using a 'shorter' antenna will cut down on what noise you might hear. The bad part is that it will also cut down on the stuff you want to hear also. That's typically NOT the way to go about getting rid of noise, although it can work. Getting rid of noise is a huge subject all by it's self. The basic idea is to get rid of the noise sources, or make the noise go somewhere else besides your radio. That's accomplished by shielding, by-passing, and such. The two ways noise gets into a radio is by the AC/DC power line, or by the antenna cuz the noise is being radiated on a radio frequency. It isn't the antenna's fault, or the power line's. You can do things with the power lines that you can't do with the antenna, which is just a matter of how things work.
Enough of all that...
- 'Doc
