After flush the toilet it makes a loud rattling noise, this happens to both of the toilets, one upstairs and one downstairs. I am scared that it will be a expensive fix and I am a single mom and do not have lot of money to spend. Does anyone know what this might be and any ideas on how to fix it?

Do you get this same noise from turning on any faucets or the ones in the bathtub/shower ?

If you hear the noise with general water flow, I would suspect the house pressure regulator.

No, it only happens with the toilets although last night even though the toilet was not being flushed it made the noise, it sounds like a ship horn.

Ahhhh! The dreaded potty monster, a fearful sound in the middle of the night. Terror in the toilet so to speak. In order to nail this down, I need a little information. Is the sound worse when the flush goes down or when the tank fills? Is this a first or second floor john? And does your water pressure fluctuate with the season? Are you on city sewer or a septic tank?Click on back with the info and we will try to muzzle that cantankerous commode : Yes, it is a dreadful sound and sometimes it lasts for a full minute!
A long time ago I had a call with symptoms like yours, Onle this one kicked in at 3 in the AM and scared hell out of the customers. It was hard the track down because the water picked up the vibration and carried it throughout the house. We call that sympathetic viberation. I want you to do this. Take the tank lid off and flush. Wait until you hear the noise and then take the float rod and push it down and lift it up gently. Does the noise change or stop? If it does then the culprit is the washer in the ballcock viberating against the seat when the ballcock is just about ready to shut off. Try it and get back to me.
: the noise changes when you move the float rod,it stops if you pull up on it. The guts are all new in this toilet. I have also tried turning the water shut-off valve way down to nothing.
The culprit is the ballcock. If you can't stop it by controlling the pressure than Shut off the shut off valve and remove the three screws holding the top and pull out the washer assenbly. Now with your hand held over the opening to prevent splash just crack the shut off valve enough to flush out any trash that may be logged in the supply tube. Reassamble and check again. While you have the lid off check the flat part of the flapper, it it has any wavey edges replace it. Also the linkage chain should have 1/4" of play when the tank is filled. If it is tighter then that loosen the linkage until you have the desired play. Thanks again for rating me. TOM

At this time it does not happen only when we flush the toilet, the horn sound just happens at random? One toilet is on the second floor the other on the first floor. It lasts only few sconds and sometimes it's louder then others, the vibrations seems ot et stonger at times. This horn sound does not occur immedietly after flushing sometimes it can be 15-30 min, so it's really puzzling. I've tried to track down the soundand hear where it might be coming from but it does not last long enough to follow. I will try your suggestions and let you know if they worked, thank you so much for the time you took to answer my question.

CLOSE the supply valve behind the toilet all the way than OPEN it all the way. Flush the toilet. See if the noise continues... Back to you... Milo