Q&A

The buzzing noise is one of your relays going bad, most likely it is an intermitten proble. You will need to test the relays to see which one is bad.

Looks like Sentras have some history of stalling problems related to a few different areas of the engine management system, none of which suggest a faulty alternator. I wouldn't think the new Alternator was the cause of your stalling issue but I have seen some weird stuff. Sentras have seen their fair share of EGR issues, Crankshaft sensor isuues, distributor issues and ECM issues all that will create the stalling issue you describe. Do you get a check engine light with the stalling issue? Most likely, the only way you will get to the root of the issue, and an accurate answer for what's going on, is through some diagnostic time with a qualified technician. Find a GOOD Diagnostic tech. (like our lead tech Eric - or anyone else with a known reputation for excellent diagnostic skills). A good diagnostic tech is a lot cheaper and a lot more productive than blindly throwing parts at a vehicle to hopefully solve a problem.
Hope this helps.
btw - the buzzing relay is easily fixed, just replace the affected relay. If the bussing has gone, your probably ok. It was most likely buzzing due to the low voltage condition created by the bad alternator.