since then, it makes a sound like a trumpet blowing. It lasts a few seconds, happens spontaneously and did not do this until he added the freon last summer. Any ideas on what is causing the noise?

 

could be the compressor going bad the 01-03 have a recall on them as when they were assembled at the factory the machine that adds the compressor oil was incorrectly putting in too little oil and they were failing if the van has less than 70K they will replace the compressor NC even if your not the orginal owner might be worth looking into