mhhh just a sugestion

var enghandle;
var idle_handle;

sound idle_sound = <Engine_idle_d.wav>; // idlesound file
sound engsound = <Car_Engine.WAV>; // Engine sound file

idle_handle = ent_playloop(my,idle_sound,20);
enghandle = ent_playloop(my,engsound,50);

first store the sound, then play it