The short answer
Here is what the symptom means on a Black+Decker microwave: a buzz from the case that only appears under load is usually panel resonance rather than a component. Pressing the case identifies it in seconds; clearing the vents fixes what usually causes it.
What you actually see
- A grinding or scraping under the floor of the cavity — A sound from below the glass tray is the turntable drive rather than anything electrical.
- The glass tray clatters as it turns — A clattering tray is nearly always seated wrongly on its coupling.
- A hum that continues after the cycle ends — A fan running on after a cycle is normal on many ovens and is not a fault.
- A buzz that has become noticeably louder over months — Gradual change is wear; sudden change is something that moved.
Causes, ranked for this microwave
- Case or cover panel loose — Ranks first because a countertop oven that has been moved often works its case screws loose, and a loose panel buzzes against the chassis under load. This is the entry most often blamed correctly and diagnosed carelessly, so take the extra minute.
- Cooling fan obstructed or its blade fouled — Second because a fan noise that continues after a cycle finishes is a useful clue, because the turntable stops and the fan does not. Confirm it before moving on, because the entries below read differently once it is ruled out.
- High-voltage section noise — Third because a microwave hums loudly under load by design, and the tone varies between models and gets no quieter with age. Rule it in or out properly here; a maybe at this position costs an hour further down.
What to check, in order
- First — the case screws — Check that visible external screws are snug. Do not remove any of them — the case stays on regardless.
- Second — the vents and what is near them — Note whether the sound continues after the cycle ends. A sound that outlives the cycle is the fan.
Checks an owner can do safely
- Step 1. Compare full power against low power — Run a short cycle at each. A buzz present only at full power is the high-voltage section doing its job.
Tools: mug, waterAbout 10 min
- Step 2. Remove and refit the tray and roller ring — Lift both out, check the ring rolls freely, wash and dry them, and refit the tray so it seats onto the coupling rather than resting on it.
Tools: noneAbout 10 min
- Step 3. Inspect the ring wheels and coupling — Look for flattened wheels and rounded coupling teeth, and spin each wheel with a fingertip to find one that will not turn.
Tools: flashlightAbout 10 min
Before you open anything
Safety
Unplug the appliance or switch off its breaker before removing any panel. The high-voltage capacitor in a microwave can hold a lethal charge long after the cord is out, and it is not discharged by waiting. Nothing inside the outer cabinet is an owner-serviceable part, and no fault justifies running the oven with the case open or with a door switch held closed by hand.
When this becomes a service call
Stop and call when the next thing to do meets any of these. Owner work on a microwave ends at the cavity, the tray, the vents and the outside of the case. Everything else is service work or a replacement decision.
Sources
General guidance- The Control of Hazardous Energy (Lockout/Tagout) — OSHA 1910.147 and manufacturer service manuals, fetched
Nearest pages
What this page does not cover
One limitation worth stating up front: this page is written for the Black+Decker microwave range rather than for your particular unit. It does not name families, model numbers or control revisions. Where a page exists that does, it is more precise than this one and it is linked under Related pages.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
