Odd sound caused by amp

[Oni 0]

So I've tried both my HU and Ipod for a source to my amp and wired the amp to 2 different subs at different impedances. Always turns out the same way, some rattling sound coming from the speaker. So Obviously it's the amp because I've tried two of everything else. The "sound" plays with the signal, not with engine RPM, so I know it's not related to my ground. You can hear it at low volume with gain turned up just enough for sound to come out. I think it's also causing my amp to go into protect mode prematurely for some reason because it would go into protect around 30 volts on the DMM when setting with a test tone.

Amp is a Hifonics 1505d (explains a lot, I know) left over from an old install in my last car. I'm planning on buying an AQ1200d because it is in my budget, but I'm curious to know if anyone has had a similar problem because I want to know what caused the problem for my own knowledge.

On a side note, I have another just for my knowledge question. Would running only a single RCA connection (only left or right, not both) to a mono-block amp affect anything like the sound quality, output level, etc or would it work the same as with both connected.

[ABXX49 20]

With no signal going to the amp, push lightly on the cone to simulate movement. Do you hear any sound when you do that?

[shizzzon 461]

if only running one of the two rcas, you half the signal voltage going to your amp so in theory, you would have to increase the gain on the amp a lot higher.

Why would it be set like that anyway?

[NB50 206]

Did you use the different subs in the same enclosure?

[Oni 0]

To answer all the questions

1.) Yes I've tested the sub, it's brand new and it's fine. No scratching metal from pushing the cone. I've also put pressure on the dust cap (Fi Q) and it didn't go away.

2.) It's not, I just saw the question never get answered on another forum and I thought that was the answer, but I didn't know for sure.

3.) The different sub was also in a different enclosure.

So that's why I feel it's the amp. But I have no experience with broken parts on an amp (First time I've had one break before I sold it) so I am just wondering what would cause this noise.