It's the sound of his CD player on some piano strings, the sound of a Xerox copy machine(at that time) and the sound of a capri(ford convertible) opening up, then he turned the oscillation down on his recording device giving that "doppler" effect. He feels like he can't use his own sounds because so many people use this free sound.
Saved you 20 boring minutes.
I guess this is IGN attempting the "No Clip" kind of thing.
Decino is great. If you want to get back into classci Doom watch his Sumlulst Playthrough. Lots of skill and very inspiring. Classci Doom with all its community content is still the best Doom imo.