1994 GMC Sonoma engine noise

We know it's not a defective sending unit because the engine noise tells us the oil pressure really is going low intermittently. Two common causes are debris in the oil pan blocking the pickup screen, and debris in the pressure relief valve in the oil pump, or a broken spring in it.

I don't think this applies to your engine, but on older GM V-8 engines, the pickup screen can swivel or pivot, and lift up enough to draw up air instead of oil. Aerated oil can compress and will result in low oil pressure. Most engine rebuilders braze the tube to the oil pump body to prevent this from happening.