weird noises rca + ground?

Hi 12volt. (kinda long post, appreciate your help)

97 VW polo (90amp alt, big3 pending)
0ga power&ground amp wire, 150a fuse
pioneer 6600bt, focal er5 RCA, dls comp (no amp)
kicker zx1500.1 - alpine type-r 10" (1000rms) - custom enclosure tuned ~36hz

Problems: A low volume hiss from sub. You have to literally stick your ear to the cone to hear it. A jet turbine like noise, something pitchy like a -digital- high rpm sound. It's tone goes up and down randomly. Engine on or off, RCA's plugged in or not, it doesn't matter.. it is there. If it is harmless to equipments, I don't care. Local shops said this sound is normal from amps.. i don't think so. Plus there is a tiny pop from sub when amp powers up.

My main concern is when RCAs are plugged to amp AND engine running, sub makes a little vibration(+little noise mentioned above) Engine off & rcas plugged: only humming. And it seems like sq went to doodie gradually.. like every note is same. -Maybe this is placebo-

When gain setting with a dmm I noticed **** volts fluctuates (ex: 44.5 - 46.7 - 45.2 ...) bought a bad amp?
+ and maybe installer damaged my HU rca outputs?

some extra info:
set gain conservatively with help of dmm & no dimming
almost never pushed the system too hard (i hope)
50hz 0db test tone, HU %75 max volume used
signal - power wires from different sides of vehicle
amp ground is good but i will make it better
Rca cables: triple shielded, remote wire is pointlessly kinda big lol.. noise from there possible?

Bad batt ground & bad HU ground can cause amp to generate noise even when RCAs are unplugged?

Thanks!

Update

When everything is running that lil sub vibration(@zero volume) disappears if I set amp's crossover to 50hz. It starts again around mid way (125hz I guess?) there is no sub crossover OFF option at my HU.. :/

pitchy digital high rpm like sound from sub is so low that IF it is harmless and doesn't disrupt sq at full tilt.. I don't care it. what do you think?

well I tested with a hifonics 1208d and same things happen.. but much more hard to hear. Whine noise is there but noticeably less than kicker, especially when no RCAs connected.

hifonics xover increases noise&volts too.. interesting, I didn't knew it had such effect. So I had to set HU lpf 200hz -24db and amp lpf ~80hz -24db is it o.k.? :crap:

my new problem is cone gets warm even though I set everything conservatively. I guess bad enclosure design