I have a whistling Quadrajet carb on a 454 marine engine. It sems to be coming from the back of the carb. Any ideas?

sounds like the float is sticking or may have a hole in it, you can try adding some automatic transmission fluid to the gas - sometimes works if it sticking,check for a vaccume leak - but usually won't overfuel with a leak- but has happend.

could be a multitude of things. Firing back through the carb usually is caused buy timing issues. The distributor has a vacuum and a mechanical advance. If the advance is not properly functioning the engine will be low on power and may fire through the carb when accelerating.

on this truck the vacuum is pretty basic, im guessing you have a 305 (5.0) or a 350 (5.7), on the front of the engine compartment near the fan shroud somewhere (if it is still intact) you should have a diagram of all the vac lines
you probably have a quadrajet carb, if you are mechanical and patient they are not hard to rebuild

blowing out the carb with carb and choke cleaner isnt a bad idea either, have someone keep it running and slowly run like 1/4 to 1/2 a can down the carb spraying all around

If you have the original engine, likely it's a Quadrajet if it's 4bbl, or another possibility is a Rochester 2gc. The Quadrajet is a modified version with a purge valve solenoid and a throttle position sensor built into it. (funny to see your post because I just took one off a Montecarlo this morning....did a vorteck head swap, cam, high perf. intake, headers and a way larger Holley carb)

weak fuel pump

Check with some of your local shops that rebuild carbs, they can probably set you up with one. A new one will have to come from GM likely. You have the best idea in going to a junkyard and cannabilizing, I think. Hope this helps.

Update? If the Motorhome has a carburetor, its pretty old and has a P30 chassis. Gm makes the truck, Honey makes the camper. Check the electric choke and make sure it has power to it when the engine is running, if not check all fuses. I dont have a diagram to see where the choke gets power from. Make sure the choke pull off is working, its a little vac can on the right side of the carb, if it leaks, it wont pull the choke flap open. Anyone that works on Quadrajet Carbs can fix it for you.

check the distribor cap and rotor buttom, sounds like a cracked cap, hope to help

almost sounds more in the carburator. if this motorhome sat for a long period of time, when you opened it up, it increased the fuel flow in the bowl of the carb, therefor may have stirred up dirt and varnish. id check there first. if it has a holley on it, if it "coughed" or backfired through the carb when you "opened" it up, it more then likely blew the power valve out

There is a vacumn hose that has come loose somehow. Use a second person to isolate the whistle to lead you to where the hose is cracked, loose, or detached. Fix or replace and this should fix this whistling sound. Hope this helps.