Recently discovered rust spots may surprise you when you spot them on the carpet, but in many cases, the furniture is to blame. Remove those suspect spots with one of several homemade stain lifters, testing each substance on an inconspicuous area first to ensure the stain remover does not discolor the carpet.
In some cases, you can loosen the rust a bit with a butter knife. Scrape the knife blade side to side through the discolored carpet area, fanning the carpet fibers to loosen dry particles. Vacuum the spot; then fan the knife through again. To help you keep a close eye on your progress, select a vacuum cleaner with a hose attachment and hold the hose near the carpet. Keep the knife in one hand, the hose in the other, alternating back and forth between the two tools until no more dry rust comes out of the carpet.
Squirt a small amount of dish soap onto a damp white cloth; then press the soapy area atop the rust stain. Blot the area several times with the cloth; then allow the soap to sit for 5 to 10 minutes. Dab a dry white cloth or paper towel over the soaped area, absorbing some of the soap and rust. Check the cloth or paper towel for rust as you dab; as it picks up rust, switch to a fresh area of the material and continue dabbing. Wipe up the soap afterwards with a fresh damp cloth.
Dab the area with the corner of a white cloth soaked in white vinegar, keeping the vinegar contained to the stained area. Press down a bit to wet the carpet fibers. Sprinkle salt over the moistened carpet; then allow the salt and vinegar to set for 30 minutes or so. Blot the area with a dry cloth; then vacuum up any remaining salt. Wipe the area again with a damp white cloth.
Lemon juice may lighten any remaining rust so it is no longer noticeable. Pour some lemon juice into a shallow bowl; then shake in a generous amount of salt. Dip a white cloth into the lemon solution; then dab it over the rust stain repeatedly. Press down so some of the lemon juice transfers into the carpet. After 5 minutes or so, rinse the area by wiping it with a wet white cloth. Pat with a dry white cloth afterwards to absorb excess moisture.
If you suspect the source of the stain is metal furniture sitting atop the carpet, check the legs of tables and chairs for rusty areas. Place plastic caps or furniture coasters under rusty furniture legs, or use a dark area rug under the furniture to prevent the rust from reaching the carpet.