hissing coming from engine and oil pressure is low
 

i noticed today that i have a hissing noise almots like a vacuum line fell off but my dad helped me check em and none fell off and we cant find where its coming from,

also today when i drove it i noticed that the oil pressure is noticeably lower than normal around 10-12 psi at idle and about 35 psi at 3000 rpm normally it ran around 30 at idle and around 60 at 3000 rpm i cleaned off the oil pressure sending unit cuz i heard that connection can get crudded up but that didnt help

if u have n e idea what could be causing these problems please let me know.
thanks