This is a common problem on some VX-5. I had it to. The solution can be found onhttp://www.ukmidlandscanner.co.uk/r2vvx5.htm
Some VX5's suffer from speaker howl when volume is turned right up this is a bug which can be fixed by following these instructions: Speaker howl: Testing: any active frequency, turn volume knob from low to high. At some point the speaker will start to 'howl' It is not a distortion of voice because of tiny speaker, not feedback, but a self-generated tone that seriously disturbs understanding anything. It only occurs with the internal speaker, not external ones. Causes: The speaker board is linked to the audio one by a thin flexible circuit where a single ground wire isn't enough to carry the current to the speaker without disturbing the reference levels, causing this oscillation. The original design had the metallic case as carrier for these higher currents, but the painters of the case never knew about it. Fix: Yaesu acknowledges the problem, so you can just send it for warranty repair. If you are a do-it-yourselfer , open the radio, and where the four screws that are in the battery compartment (You just unscrewed them to get this far) on the inside of the radio you will see a raised quarter circle around each screw hole which acts as a contact to the keyboard p.c.b. Don't file them, otherwise they will be too short and not touch the board above. Now with a medium size screwdriver lightly scrape at the top of the quarter circles just enough to give the tips a little shine. Zip the radio back up and all should be well.
This worked for me
73 de Jos PA3ALF