That sounds like it could be the
thermostat housing or possibly a
water pump. Both are located on the passenger side of the engine in a 1999 Chevy Astro and a leak from either would show up on the passenger side fender wall. A less likely but possible culprit could be a
heater hose connection near the firewall.
To be certain, you'll need to visually inspect the area. Look for wetness, corrosion, or signs of coolant staining. A flashlight and possibly a mirror to get into tight spots will be helpful.
