Robagray

My spark started making a hissing noise behind the dash board. The noise is there when the a.c. is selected to be on. When the a.c. is not selected the noise is only there when the vent selector switch is set to defrost. While sitting idle with tthe a.c. the noise is there. If you rev the engine the noise goes away for a bit then comes back. Checked the a.c. pressure and it appears to be normal when checked with bottle refill kit.