Intermittent gurgling noise when turn truck off

My vehicle is twenty years old. Prior to now, no major issues (at least I’m thinking) except the top radiator hose burst, cap and coolant tank that was replaced. It was notice that at the neck radiator where the spring clamps onto, there’s a part that had deteriorated but the hose was able to still cover. Now I’m hearing a noise that sounds like “gurgling” to me but it also sounds like a sink that’s being drained after pulling the plug out. This is only after the truck is turned off. Not while driving nor idling. The gauges except the oil gauge which upon acceleration stays around 70 (for past 20 years always) stayed at 40 driving and idling. Last problem is this beep sound that these vehicles do as indicators of low coolants and washer fluids being low. Well my vehicle beeps (intermittently) but nothing displays. I’ve had it diagnosed and mechanic tells me everything is fine. Although nothing needed top off he did it anyways. Yeah, I know, how? Right! I have had oil changed, oil sending unit installed, idle pulley, some other pulley to no avail. My plan is to have the water pump and thermostat and lower hose replaced. Am I heading in the right direction or should I be looking in different direction? I’m trying to avoid major damage and/or repairs.