Hoover 'Sensotronic'
Caution is advised when using this cleaner; the motor has a tendency to burn out. Bag had to be changed regularly; motor filter was washable.
If I remember correctly, the 'System 2'-onwards have two-stage motors. The stationary baffles between the two rotating fans, is made of thermoplastic. During prolonged use with a clogged bag (even moderately used/half full); the motor gets hot and the aforementioned baffles melt, then scrape against the fans, then get flung through the motor, and finally expelled via the exhaust diffuser.
I should know; it happened to our's. Twice.
Fan/armature assemblies were available at reasonable cost, as opposed to the complete motor.
Bag check indicator was rather useless; it never indicated the bag was full; would not register when cleaner was stood on end (magnet/Reed switch). It only registered a clogged hose or tube.
Wasn't there also the 'System 25, 35, etc'?
Ozhoover: 'Audio 300'
That's the cleaner I saw on BBC tv's "Tomorrow's World" science programme, circa 1987/88. The presenter was using it, then it conked out, played a short tune, then uttered "Hose is clogged, clear the blockage." He then dislodged the bag, closed the lid and "Bag is not correctly fitted" was spoken.
There are apparently 4 spoken phrases; the two aforementioned, plus "Bag is full" and "Have motor checked by qualified service agent" (or words to that effect).