Any cheap *really silent* AF prime lens out there?

Hi folks,

I'm looking for a new prime lens for my Canon EOS 70D with the following three characteristics:

• Standard 50mm-equivalent focal length (that is to say, something in the range 28mm - 35mm)
• Large aperture (f/2 and above, though I may consider f/2.8 if necessary)
• Low budget ($600 max)

I'm not a professional and my typical shooting situations are very generic:

• Indoor, low-light portraits / footage
• Outdoor natural landscapes

To these, I would like to add some very rudimentary astrophotography.

Now, keeping this in mind, I elaborated the following short list:

• Canon EF 35mm f/2 IS USM [Pro's: IS, Ring-Type USM / Con: price]
• Canon EF 28mm f/1.8 USM [no particular pro's or con's]
• Sigma 30mm F1.4 DC HSM [Pro: very fast / Con: no DoF scale, only for APS-C/DX]
• Sigma 28mm F1.8 EX DG Aspherical Macro [Pro: has aperture ring / Con: no HyperSonic Motor]

Now what is really getting me stalled is the noise issue when AF'ing.

I know, I know: I should not use AF when taking footage, and I can't even think of using the internal mic, can I? But...

But I'm not talking about taking beautiful and/or professional footage here, I'm talking about filming my boy while he is playing (typically indoor, i.e. in low light). That means that

1. I cannot use an external mic (I have no time to mount it, and I'm pretty sure my 3 yr-old son would claim that's one of his toys anyway; and, how can I be sure that an external mic would really get rid of noise?),
2. I need to use a fast aperture, thus I'm very dependent on AF as I
3. have no way to focus manually (I need to pay attention -- a lot of attention! -- to the subject)
4. cannot rely (for sure!) on the subject standing still.

You may say that I may use a smartphone for that as well, but hey!, I own a 70D, I want to use it!

So, back to this noise issue, I thought that

1. STM is the best
2. Ring-Type USM is nearly as good as STM
3. USM (which is HSM in Sigma's language) is fairly good
4. older non-UltraSonic Micromotors just suck.

Now while on the 4th point there is no doubt (I own the Canon EF 50mm f/1.8 II, which is indeed pretty unusable for videos with AF), I thought the same for the 1st, but it turned out this was not the case (see the noisy pancake Canon EF 40mm f/2.8 STM, with a test here). So I have the feeling that every lens has its own performance in terms of noise when performing under AF. Is that correct?

Btw I guessed "oh well, I'll go for the Canon EF 35mm f/2 IS USM, which has Ring-Type USM". But it turns out that audio becomes inaudible when IS is on -- which basically makes IS useless when taking videos from my perspective. Also, I have no proof whatsoever that audio is good even when IS is off. Maybe someone has some experience to share with this lens?

And, more in general: any help?

Thank you

Sergio