I'd pick either the hornet or the viper I like them both, and the fact that we even have a slight chance of seeing DCS: F/A-18 soon is a great thing :)
On the Russian forum, most want to wait for EDGE before the MiG-21 is released, and I feel the same way to some extent, but if it comes out in 1.29 (hopefully) then I'll be more than glad to get it :)
Does it start when you do the startup tutorial mission? Just follow every step and make sure you hold the switch to the left of the starter for a few seconds after you do the primer.
Is this when going just over Mach 1? This is intended, because you are flying at the same speed as the sound wave from the gun, which means you will hear it multiple times.