What A/C Did I Just Hear

Been an aircraft enthusiast since mid-sixties and know most if not all types by sight and sound.

I have lived and worked several times at or under runway approaches to three military fields and two commercial fields in various states.

I usually have no trouble at at telling an aircraft type just by sound.

Last several months I've been rural in east central Alabama which seems to be common corridor for low level treetop practice flights by C-130's, FA-18's, F-16's, Apache's and Blackhawks.

Tonight had some type of aircraft over that I've never heard in all my decades of listening.

The aircraft was certainly at 250-500 feet going by sound doppler change and made a very low hum yet I could tell it was jet powered as it still had slight turbine pitch.

Sound wise, closest comes to mind 10% decibels of an FA-18 with half the low pitch hum of an A-10 but not the whine at all.

Any ideas?

My thoughts were at first maybe a drone but it was to low to be much use for cameras and sensors, flyby was all of three or four seconds.

Reason I'm posting this is sound was very unusual and it was very low level night flight that followed previous mentioned tracks by multiple other identifiable military aircraft.

a reply to: Phoenix

It could be anything. Alabama is home to one of the high speed corridors where they do testing of aircraft, as well as exercises with units from all around the area.

a reply to: Zaphod58

Thanks Zaph,

Yup could be anything, however, it's nothing that is in "normal" inventory as I know how those all sound - this was entirely different aircraft I've never heard before.

Sorry there was no time to get any pics nor actual sighting.

Definitely was not any regular fighter, transport nor helo usually seen or heard here or elsewhere I've been.

Think hum from amplifiers combined with deep bass turbine sound - guarantee that bird went silent in less than 1/4 mile after being overhead.

Nothing compares in 50 years of paying attention.

a reply to: Phoenix

There are quite a few that sound different than you would expect sometimes. Perfectly normal aircraft, just sometimes the engines sound different than you'd expect. All the good stuff is tested somewhere other than Alabama, usually out west.

Two or three times this year I had a group of the V-22 Ospreys fly over the house. I get a lot of different traffic over my house and those have some of the strangest sounding aircraft. I would almost describe it the way you have in your description. I don't remember what mode of operation they were at the time, but I believe they were in "airplane" mode.

a reply to: DinobotIV

Seen demo of Osprey at airshow in "airplane" mode and that wasn't it last night.

It was like a deep deep hum with turbine sound in it, but definately not turbo prop sound.

Going by FA-18 flyover at treetop and guessing their speed at 350ish knots and comparing doppler change to last night's flyover I'd guess speed was 2-250 knots.

Mentioned in beginning post that I can identify 99.9% of both civilian and military aircraft just on sound alone, makes no difference if they are in climb, cruise or descent, I do this all the time.

What I heard last night was nothing in the "normal" inventory.

Need some conjecture so I can go in right direction in identifying as I'm stumped.