On the GP35 you just want DCC or sound and DCC? If DCC use the NCE decoder made for the Bachmann GP35: https://ncedcc.zendesk.com/hc/en-us/articles/200945019-BACH-DSL
On the FT you need to know if the motor is isolated from the frame. If it is, the NCE decoder might work as well.  Joe

Write CV 2 to a value of 0. Try that. That should make the loco start out on speed step 1. If not increase the value by 1 until you get what you want. You may want to replace the decoder as the ones Bachmann uses aren't that good. If it's the one with an 8 pin plug, TCS makes great decoders. NCE makes a board style as well.  Joe
NCE    https://ncedcc.zendesk.com/hc/en-us/articles/200945019-BACH-DSL

Run the B unit by itself to see if it makes the grinding noise. If not it sounds like it's running faster then the A unit. Put both on the track about a foot apart. Run them at various speeds.
if the B unit catches up to the A unit, that's  the problem.  I don't know how well the Bachmann decoders can be speed matched. So 2 problems: you can't speed match using you current system as it doesn't program cv's and the decoders aren't that good. I suggest replacing the decoders with something better. NCE makes a good decoder for Bachmann DCC on Board locos. That leaves you with finding someone to program the locos for you.  Joe
NCE decoder:  https://ncedcc.zendesk.com/hc/en-us/articles/200945019-BACH-DSL

Are they DCC ready? If they are they should have an 8 pin plug for a decoder. If not, you have to hardwire them in making sure the motor is isolated from the frame. You may want to check out the NCE decoder for Bachmann locos. These are a much better decoder Joe

https://ncedcc.zendesk.com/hc/en-us/articles/200945019-BACH-DSL

 Another fix would be to cut a gap in one rail just to the right of the frog. Use a saw or cut off disc. I have Atlas turnouts and all of them, I gaped one rail on the diverging side of the frog and added a  power wire past the gap. No problems at all.  Joe

Does it have a decoder with a pig tail with an 8 pin board on the end that is plugged into the board or just a board? If the board has an 8 pin plug you need an adapter. If not you need to remove the board and hardwire the decoder.   Joe
Adapter:  DHWHP 9 pin to DCC medium plug long harness or DHWHPS 9 pin to DCC medium plug short harness
http://www.digitrax.com/products/mobile-decoders/?page=2
  Joe

You can use the Sountraxx booster PTB100. I use the Digitrax PR3 with a 16V power supply. Digitrax has the PR3 Xtra which comes with a 14V power supply.   This allows me to use Decoder Pro which is free. Read up on both.    Joe
http://www.digitrax.com/products/computer-control/pr3xtra/
http://jmri.sourceforge.net/

CV's 128,129,130, and 131. CV128 is master volume,CV129 is horn volume, CV130 is bell volume and 131 is exhaust volume. On each, 255 is max value. I use the 6 foot rule: 6 feet from the model, the sound should be barely heard, the bell should be the loudest followed by horn the diesel sound. You will need to experiment with different levels for each sound.  Joe 

What decoder is in it?  Joe

look at a Digitrax PR3xtra. It's a computer interface. You hook it up to a PC through a USB port. It comes with the install cd and power supply. You then download JMRI which is free. The PR3 hooks up to a programming track, JMRI reads and writes CV values to the decoder. You keep a file so if the decoder gets re-set, just reload the values using decoder pro.  Joe

Soundtraxx new Economi decoder. It can be hardwired in or have an adapter to plug into an 8 pin socket.
http://www.soundtraxx.com/dsd/econami/eco100.php
They also have a board style.
http://www.soundtraxx.com/dsd/econami/ecopnp.php
Joe

 Put the screw driver on the screw head and give it a whack with a small hammer. This might help loosen the screws.  Joe

If you can't find a Sprog, the Digitrax PR3 extra will work as well.  Joe

Where the point rails pivot, is there a grommet that holds them in place? If so drill a hole through the hole and road bed. Put a feeder wire through the hole and hook up to the track feeder. Do this to both rails. Also solder a small piece of wire from the grommet to the bottom of the point rail. If the frog is metal, gap one of the rails that come off the frog end on the straight or diverging end and put a feeder wire on the rail past the gap.  Hope this helps.  Joe

Which digital controller do you have? If it's the Dynmis you can program CV's with it. If it's the EZ DCC you can't and need to find someone to program for you.   Joe