Two pronunciations of ങ്ങ

In Malayalam there are two possible pronounciations for ങ്ങ. 

In words like മാങ്ങ, പൊങ്ങി, the pronounciation of ങ്ങ is very 'pure' - it sounds exactly like two ങ's. Whereas, in words like തേങ്ങ and മുരിങ്ങ, the pronounciation is more like 'ങ്യ' (ങ് + യ ). 

Is it completely random, or is there a rule behind it?

It seems that when the preceding sound is gutteral (e.g. അ, ഓ or ഉ), the pronounciation is the pure one. When the preceding sound is palatal (e.g. ഇ, ഏ), the pronounciation is mixed with യ. 

For a set of contrasting examples, consider: ചങ്ങാതി (1), ചിങ്ങം (2), ചുങ്ങി (1), ചേങ്ങില (2), ചൊങ്ങ്‌ (1), where (1) denotes pure and (2) denotes impure pronunciation. 

Can you think of other examples that demonstrate this difference in pronounciation?