The neighbor's HVAC compressor was replaced

[WhoIsThat?]

and now it starts out loudly but goes down to normal noise level in a half minute or so.
What is probably happening to cause this?
TIA

[mark beiser]

It could be refrigerant migration causing liquid refrigerant to flood the compressor when it starts.

Possibly what killed the old compressor...

[suzook]

How loud is it? It is normal to be slightly louder when it 1st kicks on.

[WhoIsThat?]

How loud is it? It is normal to be slightly louder when it 1st kicks on.
The drop in sound is unlike any compressor I've heard. The contactor whack sound is normal but mine immediately goes into a purring sound.

What cause refrig. migration?
Never mind; is the cure
". . .the use of an automatic pumpdown cycle."

The HVAC guy was called back several times and at least once he suspected the start capacitor. Of course, it started to rain while he was working on the thing.

I'd help the neighbor with interpreting what I hear but I hate his guts. Let him pay for another new compressor.

[Shophound]

Let him pay for another new compressor.

He probably will, since so often the cause of the prior compressor failure is never investigated and resolved.

I'm with Mark, I suspect off cycle refrigerant migration into the compressor. Liquid refrigerant collects in the compressor oil sump during off cycle, and flashes off once the compressor restarts. That takes oil with it, so the compressor is now not only pumping vapor, it's pumping oil droplets into the condenser. A crankcase heater can mitigate this problem. Liquid line solenoid also is effective, but requires modifying the condenser controls so the compressor does not turn off when the thermostat satisfies, but shuts off when the liquid line solenoid closes and the system pumps down to a preset suction pressure. Not hard to set up for any tech not shy around controls and control wiring.

System could also be grossly overcharged.

[WhoIsThat?]

All good.
I'll read up on migration and wait for this sucka' to die [the compressor, that is. . .].

[Irascible]

Thirty seconds of noise from slugging? I'd reply with "highly improbable", but given the replies from my esteemed colleagues I'm not so sure. You two are still esteemed, no? ;) I kid of course, but what's up with a half minute of liquid?

[Shophound]

Thirty seconds of noise from slugging? I'd reply with "highly improbable", but given the replies from my esteemed colleagues I'm not so sure. You two are still esteemed, no? ;) I kid of course, but what's up with a half minute of liquid?

Didn't have the time increment mentioned in the OP in mind when I replied. May have subliminally, since I mentioned possible overcharge. With it now in my conscious memory, I'd lean even more that direction. It's possible what the OP is hearing now may also have been going on with the prior compressor, but no way to know for certain from here.

[mark beiser]

Thirty seconds of noise from slugging? I'd reply with "highly improbable", but given the replies from my esteemed colleagues I'm not so sure. You two are still esteemed, no? ;) I kid of course, but what's up with a half minute of liquid?

Well, the OP didn't say "30 seconds", he said "half a minute or so".
For all we know it could be more like 10-15 seconds or less, but to someone hearing their neighbors noisy AC start up, it may subjectively seem longer.;)

I just filtered out the "half a minute or so" part and went off the complaint that it makes a noise on start up that quiets down to normal.

Since none of us have heard the system, all any of us can do is throw out possibilities anyway.

[heaterman]

Well, the OP didn't say "30 seconds", he said "half a minute or so".
For all we know it could be more like 10-15 seconds or less, but to someone hearing their neighbors noisy AC start up, it may subjectively seem longer.;)

I just filtered out the "half a minute or so" part and went off the complaint that it makes a noise on start up that quiets down to normal.

Since none of us have heard the system, all any of us can do is throw out possibilities anyway.

Guess he could sneek out and record it and You Tube it. Give Twilli something to listen to.

[WhoIsThat?]

It might be loud at a constant level for 10-15 sec following the contactor "whack" and then taper down to normal in the next 10 sec. I keep expecting it to get louder and then die, but it never does.

Now it's so hot/humid here that we're running our own central air so with the windows closed we don't hear it much. Our compressor has the whack and than a purr, like it has for years.