Is it safe to drive with a grinding noise?

When the grinding happens tells you what's failing, because grinding is always metal contacting metal that shouldn't be. Grinding specifically when braking means the brake pad friction material has worn through and the metal backing plate is contacting the rotor — fix it immediately, it's a safety issue. Grinding that gets louder turning in one direction is a wheel bearing failing (usually on the opposite side to the way that makes it quieter), or a CV joint. Grinding only at startup that goes away once the engine runs is the starter motor clashing with the flywheel ring gear. Grinding constantly while driving straight, not tied to braking, is a wheel bearing or a brake caliper dragging. Grinding when shifting gears on a manual is the clutch or a worn synchronizer. And grinding that appears only when the AC is on is the AC compressor bearing. Note exactly when it grinds, isolate which wheel or system with the free tests below, and you'll usually name the component before a shop touches the car — which is what keeps a $150 pad job from becoming a $500 rotor-and-pad job.

Grinding specifically when you press the brake pedal — a harsh metal scraping that appears or worsens under the pedal — means the brake pad friction material has worn through completely, and the steel backing plate of the pad is now grinding directly against the iron rotor. This is one of the most urgent noises a car makes. Metal-on-metal contact destroys the rotor surface fast: what would have been a $150-$300 pad replacement becomes a $250-$600 rotor-and-pad job within days of grinding, because the rotor gets scored past the point of reuse. There's one benign exception — a light grinding or scraping on just the first one to three stops after the car has sat overnight in rain or humidity is usually surface rust being scraped off the rotors, and it disappears by the third or fourth stop. If the grinding continues past those first few stops, or happens every time you brake regardless of weather, it's worn pads. Stop braking hard, drive gently to a shop, and have it addressed the same day.

Grinding that gets louder in one direction more than the other is the wheel bearing's most diagnostic symptom. The failing bearing is unloaded when you turn away from it and loaded when you turn toward it, so the noise changes with steering. The free swerve test at about 50 mph pins it down: on a clear road, gently weave left and right and note which direction makes the grinding louder — louder when turning right generally points at the left bearing, and louder turning left points at the right, because weight shifting off the bad bearing changes its noise. A CV joint is different from a bearing: CV grinding shows up specifically at full steering lock, at low speed, in parking-lot maneuvers, and it tends to click or pop rather than grind continuously. So directional grinding at speed is a wheel bearing, while clicking or grinding only at full-lock low-speed turns is a CV joint — two different repairs the timing tells apart. Confirm the bearing with a jack-up wobble-and-spin check before replacing anything.