Why Oracle DB Server is Heavly Swapping?

Hey guys, how are you doing? Hope it's ok! ;)

Here I'm facing an annoying problem with a legacy Server (Standalone Database running 11.2 on RHEL 6). From 10 days ago, the monitoring is freaking out with frequent alarms of high Swap consumption, and checking that, I've found nothing strange:

-- Memory config (ASMM):

SQL> show parameter sga

NAME   TYPE VALUE

------------------------------------ ----------- ------------------------------

lock_sga   boolean FALSE

pre_page_sga   boolean FALSE

sga_max_size   big integer 20G

sga_target   big integer 15G

SQL> show parameter pga

NAME   TYPE VALUE

------------------------------------ ----------- ------------------------------

pga_aggregate_target   big integer 4G

SQL> show parameter memory