It’s an old sound bug. No idea what causes it.
I’m moving this to the main report thread about the issue.
//Mod
Ah yes, I noticed that, too. But this time it’s mostly Lynx engine sounds.
First time I noticed that again was while looting a Wolf and suddenly an engine started nearly next to me… 
Nearly jumped off my chair until I noticed that it was only the sound… again… 
I just had two situations where I was able to find a solution to a possible ghost-machine. Once there spawned a hunter inside a house that I wasn’t able to enter… But the hunter was able to shoot out.
And once it was a tank which spawned in the basement of a special barn.
All others may be possible spawns where the system recognized that the player is too close and where it tried to stop spawning… But didn’t stop the sound.
Another solution could be, that the machines spawn inside the terrain, but I guess there would be more active sounds if you get close. They probably would try to attack the player.
As there were some situations where machines suddenly spawned directly next to the player I would choose the first solution.
Therefore: the spawn-behaviour is somehow broken.
Experienced this bug a few days ago when I started doing the FNIX Rising missions. I heard the sounds of a Tank in the ground at the Ringfort.
During the wave defense mission, a Tank did come up and join the fight, but I think this video was taken before that.
It’s grumbles from the grave.
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Happens to me all the time. Pretty much every time I play. I assume the machine spawned under the surface. Only once did it actually pop up beside me. Good thing I was wearing my brown pants. 
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so ive noticed that theres a sound glitch after you kill a tank youll here the noises he makes like the long deep groan they make. then theres the sound of like someone running next to you glitch and no ones there . then theres a glitch where my friends will see a tank or a hunter and they dont render in for me so i cant help fight them im on the pc platform
Been an issue for a long while now. I’m a strong advocate for getting this bug fixed, so I’m merging threads to my old report to bump it up. Please make another report (or search to see if it exists already) about the desynced machines.
//Mod
I like to think of it like thay have a spirit and I just released them and now Thay are angry or that it’s the people thay have captured and forcibly uploaded their mind into the rivel machines we are fighting and thay are thanking us for freeing them.
subterranean spawns happen all the time, and wolves are the worst because the sound carries for such a long distance.
its been happening for 1 year pretty much. I don’t report it because sometimes it stop happening sometimes not. not only audio from dead and despawned theyre louder.
It’s been since I started playing GZ.
Sometimes more often, sometimes less. May depend on the locations and the terrain there.
Most times I ignore the sounds.
I can’t kill what I can’t see.